Female Marines Article Discussion

[Doctor Thunder]

Okay, if there is anything we've learned from the Librarium Debate on Female Space Marines, it is that many people are soured over time to the idea of female space marines because it is often done very poorly.

 

So, to correct this problem, Ferrata and I are collaborating along with Master Jeridan to make a list of good female space marine origination ideas. This will be compiled into a list to be used as a resource for board members who want to include female marines in their armies in a believable way.

 

The title will be: How to make Female Marines...If you must.

 

So, we need to collect as many good ideas as we can. Since this board is filled with intelligent and eloquent posters, I'm sure we can get some great ideas together that fit believably into the 40K framework. The format is to give the Origination Idea, an Overview, and then address any Points on how to make it good.

 

Here's an example:

 

 

Origination Idea: Fabius Bile *scratch one - Ferrata*

Overview: As the self-styled Primogenitor of Space Marine Creation, Fabius Bile has both the knowledge and resources to create female marines.

Points: If you are going to use Fabius as part of your origination story, you'll need to come up with a reason why he would do so in the first place. His motivations should be central to the story, and there should be a lot of terrible mewing failures before success in finally achieved. Obviously this idea works best for Female Chaos Marines, but you could use him for loyalists if you take the angle of his creations getting loose and following their own path. Even then, they would be hunted by the Imperium, and so would be renegades at best.

 

 

 

So, let's see what you guys got. List as many good ideas as you guys can think of.

[flintlocklaser]

Heck, I'll take a stab at it. I'm not a big female marine fan, but I've got no problem with people wanting to try something like this. Maybe these will help:

 

Origniation Idea:The Missing Primarch premise *scratch four*

Overview: One (or both?) of the two Missing Primarchs was female, and her gene-seed could only be used to make female Marines.

Points: Two fluff "don'ts" that actually kind of work together. If you're willing to step outside established fluff in the first place, tying your Female Marines to the Missing Primarchs is less of a stretch. Plus there's always been that mystery of why it is that having half the Legions turn to Chaos could be (relatively) common knowledge, but all the details of the two Missing Legions had to be hushed up for some reason. The background of 40k is more all-around misanthropy than misogyny, but a Big Conspiracy against the female Legion could be part of the reason for the biggest, most successful cover-up in ten millenia. You'll have to come up with some reason for why they've gone totally unnoticed for those ten thousand years, however.

 

Origniation Idea: Parallel Development, aka 'We're not Adeptus Astartes but we're just as good'

Overview: Everything was better in the Dark Age of Technology! Who's to say that at some point during that long-gone golden age, some splinter human faction out there didn't create its own army of augmented warriors. Such fighters wouldn't be Space Marines in name or 'fluff' terms, but based on the snippets of detail from the Horus Heresy books and other sources, it's clear that there were other societies capable of making warriors that could give the Astartes a run for their money, at least at a one-on-one level. And there's nothing saying these alternate warriors had to be all-male!

Points: It's 'counts as' on a grand scale. Due to the current 'history' of the Imperium of Man, this idea really works best for a pre-Heresy setting; however there are some cracks in the structure of the Imperium that could be arguably be large enough to hold a seperate human society that still has access to their old pre-Collapse technology and can still produce some amount of their Marine-analogs (hiding out in the Halo Stars or what have you). A positive is that since these hypothetical warriors aren't actually tied to the Space Marine fluff, you'd have a much broader modeling canvas available to you. That's also a downside though, as for many people the big draw of playing Space Marines is their visual design. Plus it's another origin story that ties your creation to either getting the stuffing knocked out of them by the Imperium back in M31, or to being around today but having to hide like rats in the walls.

[Ferrata]

I'll get working on something for this later...

[Captain Bog-Bot]

In regards to flintlocklasers post, specifically the parallel development idea is the more fathomable option IMHO. The problem I see, and I've had this discussion with a buddy who DIY'ed a chapter with the missing matriarch was her actual gene seed. As we all know space marine gene seed keys into the testosterone spike during boys puberty, it caplitalizes on the fact the males DO get bigger and stronger as the prinicipal effect of puberty, alongside our sexual development. Whereas females, yes they grow bigger and stronger than in childhood, they're changes focus on developing the capacity to be impregnated,give birth, and sustain life until the child can be weend off of milk alone. Where does gene seed tie in with a girls estrogen increase instead of testosterone? Maybe since females do have small levels of testosterone in them (I think, just like males have small levels of estrogen right?), you could say the same results could be achieved with female Marines, with their strength, size, and speed being proportional to Marine males as normal females vs. normal males? Or should I even nit-pick this hard with our science-fiction (heavy on the fiction:o)) hobby?

[Doctor Thunder]

Maybe since females do have small levels of testosterone in them (I think, just like males have small levels of estrogen right?), you could say the same results could be achieved with female Marines, with their strength, size, and speed being proportional to Marine males as normal females vs. normal males? Or should I even nit-pick this hard with our science-fiction (heavy on the fiction:o)) hobby?

Good question. In my mind, a Female Marine would have less physical strength then her male counterpart, and would also be shorter. She would also be quicker, have a higher pain threshold, longer lifespan, and superior ability to track multiple threats simultaneously. None of these differences would be great enough to require stat changes, but the presence of a trade-off in abilities makes it feel more in character with the fluff.

 

I'll probably add a not about this in the article.

[Barret]

I think one thing that's missing here is, of course, Chaos. I was roughing out an idea (an excuse for the modeling, really) about female Chaos Marines with Molotov. I wanted a Khornate feel, based loosely on the Erinyes (aka Furies) and one thought we had was that this warband of Berserkers foiled some plot of Slaanesh's and got mutated into beastial women for it.

 

Of all of the Chaos powers, Slaanesh is the most likely to create or mutate his followers into a female aspect or appearance for obvious reasons. Tzeentch might do so also, for much less obvious reasons suitable to the Great Deceiver. Nurgle I doubt would be bothered by gender, and Khorne cares not from where the blood flows so long as it flows. Of course, both of the last two might well bestow their gifts upon a band of female warriors or cultists who, with the appropriate equipment, could be equivalenced to a Chaos Marine.

 

 

After Chaos, there is the Xeno. One of the Cursed Founding (the Subjugators, I think?) lost a small group to alien possession, and the DNA warping effects of genestealers are well known. What if a little-known, mono-gendered species (the asari of Mass Effect come to mind, but creepier) reproduce by infected and warping hosts? A group of Marines are infected, mind-controlled and mutated into looking like the alien race who, coincidentally (or not) look like humanoid females. Or what if that same mono-gendered species simply harvested Marine equipment and started using it, their alien physiology making up for the genetic engineering done on proper Marines.

[Eiric Hakon]

Maybe since females do have small levels of testosterone in them (I think, just like males have small levels of estrogen right?), you could say the same results could be achieved with female Marines, with their strength, size, and speed being proportional to Marine males as normal females vs. normal males? Or should I even nit-pick this hard with our science-fiction (heavy on the fiction:o)) hobby?

Good question. In my mind, a Female Marine would have less physical strength then her male counterpart, and would also be shorter. She would also be quicker, have a higher pain threshold, longer lifespan, and superior ability to track multiple threats simultaneously. None of these differences would be great enough to require stat changes, but the presence of a trade-off in abilities makes it feel more in character with the fluff.

 

I'll probably add a not about this in the article.

 

That's interesting: I could buy that. Someone will, without doubt, come along and say that the Testosterone levels need to be considerably higher (their proof being that marines are recruited in their teens).

[Asmodeus' Swordhand]

the fact is, so long as the IA is well written, incorporates believable reasons, and doesn't crush any other fluff in the process, anyone will grudgingly accept it. However female marines alone are enough to make some people very angry, and anyone who sets out to write a female marine story should be aware of this: no matter how loved and accepted your chapter is, angry people will shoot it down. No matter how good your reasons are this will happen, so don't be sad, just ignore these people.

 

One other thing I must add here. If one plans on making female marines. DO NOT have them working with male marines. Male marines would most likely be hostile to the female marines, and might even react with open violence towards them, for the obvious religious reasons.

[StormTAG]

What's about one of the Chaos God's going about transgendering a warband enmass? I can imagine Slannesh feeling that swapping a marine's gender woudl be hilarious. Or, perhaps, a Tzeenchi spell gone wrong ala the Rubric but with gender?

 

...This is sounding like a comic from El Goonish Shive now...

[Ferrata]

Okay, I got a little distracted and wrote an introduction to the article over anything else.

 

We’ve all seen the threads dragged down into the fiery pit of doom just by the mention of Female Space Marines, if you haven’t then you are lucky. The debate itself has been entombed within the walls of Librarium and can be found here. Now that the debate is over, we’ve turned our attention to something constructive, something to keep ourselves, something for the good of humanity (maybe a little exaggeration.) If you don’t agree with female marines this thread isn’t for you and you can stop reading right...about...now.

 

What the debate revealed is that if something is done well it is more likely to be ‘accepted’ then something done without skill. Here we offer our advice on the different plot hooks commonly used by the community to create female space marines. First we need to lay the groundwork for the project, the underlying assumptions we make to allow everything to run smoothly. The Index Astartes article ‘Creation of a Space Marine’ contains the following line;

 

'They [the recruit] must be male because zygotes are keyed to male hormones and tissue types.’

 

That is what you need to overcome to get your Space Marines of the female variety. There are two ways to nullify this statement, an in-universe one and an out-of-universe one. Let’s take the second one to begin with.

 

An outside of the (40K) universe decision that the above line is either out-dated and over-ruled by the line not being in the latest codex, that all fluff is meant to be fluid, propaganda and half-truths if you may, and it is quite likely that the above line is nothing but a lie, or finally just you taking the choice to ignore this piece of fluff, something you are entitled to do. Where does this leave you? Well, you can skip the whole section on making gene-seed female friendly and get right into the meat of the chapter creation. This method does require a small prelude to any article you produce, a simple ‘I know female marines are against the fluff but I’m ignoring it.’ Truth be told, nearly 98% of all people who dislike female marines will either stop reading there or give constructive feedback on your chapter ignoring the fact they are female. Unfortunately, there are always a few that will still yell and shout at you for it but you’re going to have to live with that.

 

The first option is a lot more complicated, as you take the quoted line to be true, and thus require some background to be produced that makes female-okay gene-seed. A large portion of this article will be given over to such methods so I won’t go into any detail here. The main problem with this approach is that you are about to walk over a mine field as many of the hooks have massive holes in them or are just damn difficult to do well. It is quite easy for your chapter to come across very cheesy, very forced and not enjoyable to read, the worst thing an article can be. We hope we can help out here by pointing out the mines before you do the largest splits you’ve ever done.

[The Nephilim]

I love it. I'm might change the Nephilim from fallen sisters to Fabius Bile created Female Space Marines.

 

One thing you guys might want to look at is some of the MechWarrior fluff. Granted, it's a different universe, but I believe their creation of Elemental pilots might hold some interesting ideas for you guys.

 

Here's some wiki links: http://en.wikipedia.org/wiki/Clans_(BattleTech) and http://en.wikipedia.org/wiki/Elemental_(BattleTech)

[Doctor Thunder]

Great intro Ferrata. Here's some more ideas to keep things rolling

 

Origination Idea: Cursed Founding Experimentation*scratch five*

Overview: The heretical experimentation done during the Cursed Founding lead to a geneseed line that is undifferentiated and can work in either gender, and someone has decided to unseal the vaults and begin using it.

Points: If you use the Cursed Founding, you need to keep in character. Your army should be plagued by mutations, mishaps, and horrifically bad luck. This is a great story hook for pirates and renegades, who fight against the enemies of the Imperium even as the Imperium itself hunts them. You'll need to come up with a person or group with authority to gain access to the geneseed, as well as a good reason for them to start using it instead of regular geneseed. (Perhaps a side effect of the geneseed gives them an ability no one else could have, like an immunity to demonic influence, or a Wolverine-like healing ability, or something like that.)

 

 

Origination Idea: Unusual Planetary Genetic Heritage

Overview: A world within the Imperium has an unusual genetic lineage, perhaps due to mutation, perhaps due to ancient mixing of bloodlines with other species. Whatever the reason, the women of this world have a body chemistry and tissue type that allows male geneseed to work within them.

Points: You'll need to explain why the authorities would use the women of the world instead of the men in the first place. (Perhaps the men are genetically stunted in their development, both mentally and physically, like the Escher gangs) Normally this is not something that Imperial Authorities are going to dare attempting, so your world will need to be an isolated radical research world, a world within the Eye of Terror (or some other bizarre phenomenon), or a rogue world that has seceded from The Imperium.

Unlike other origination stories, where the female marines could be very feminine or very masculine, depending on the author's preference, this origination scenario basically demands that the women end up extremely butch after their transformation. This is because the geneseed itself is not being modified, so people will naturally expect that it ends up with the same basic results.

[Asmodeus' Swordhand]

I like that first one...good stuff could come of that.

[Heru]

Personally if I were inclined to go about a Female Space Marine equivilent I'd run as far away from the Space Marines as possible. It wouldn't involve anyone tampering with Geneseed to make it work in women or anything like that. It would involve someone starting from scratch.

 

A number of ideas would be:

 

1. Someone makes something similar to geneseed that has some similar properties but many more that are different than those granted by Space Marine Geneseed. Most importantly this thing (whether some kind of Xenos parasite or artificial organ) wouldn't be created by messing with Geneseed.

 

2. Genetically modified female clones - perhaps a weird project that involves splicing Human and Ogyrn DNA.

 

3. Bionic replacement with perhaps just the head / brain being left intact (ala Major Kusanagi - Ghost in the Shell).

 

4. Hi-tech armour - perhaps a lost Human colony who has better than Imperium made Power Armour that gives the wearer Astartes equivalent strength, resistance and reflexes.

 

 

But then that's how I'd go about it.

[Ferrata]

Stop Now!

 

This is not a thread about whether we should be making female marines or not. I will not have that discussion again. I sent most of your replies across the Librarium, and hopefully they will merge it with the debate over there. I'm afraid Forgeman's last post was made after I sent it across.

 

Anymore discussion like what happened will see this thread closed and the project taken to PM's.

 

On a project note, next week I will attempt to write all the above ideas into a more flowing, essay styled version. If people keep coming up with the brief ideas like Dr. Thunder suggested it would be a great help.

[Doctor Thunder]

Here's a couple new ones:

 

Origination Idea: Sorcerous Curse *scratch two*

Overview: The fickle wills of the Demons of Chaos are incomprehensible to mortals. Any attempt to understand them will only warp a mortal's mind, giving the demon influence over him. Those who submit themselves to chaos risk much, for the blessings of chaos are as random as the gods themselves. As a reward, or perhaps a punishment, a group of chaos marines have been cursed with new female forms.

Points: If you are going to use a chaos curse, then your story will need to focus on the tale of how the curse came to be, rather than the science of it, like other origination ideas. The better the story the more believable it will seem, so this method is largely dependent on your skills as a narrative writer, but there are a few guidelines we can give. The more random the curse seems, the more believable it will seem. Rather than them all being changed into females, have some females, some males, some hemaphrodites, some human-alien hybrids, and some mewing spawn. You'll need to write about their reaction to being cursed, with some committing suicide rather than continuing on in their cursed forms, and others embracing it. A crusade to end the curse is always a good driving force for such an army.

 

Origination Idea: Slaaneshy Cult *scratch three*

Overview: Slaanesh offers eternal pleasure, but delivers eternal frustration, as her jaded followers search in vain for more and more extreme ways to excite their ever deadening senses. One group of chaos marines sought to alter their bodies so that they could experience new areas of debauchery previously closed to them, and found more then they bargained for.

Points: This idea works better in the small scale, as it is more believable for a couple of individuals to seek out such a change then it is for a whole group of them to do so, but if you build the story around a cult devoted to seeking out such things, then it can still work. Newer cult members can still be male, with full members becoming hemaphrodites then finally full females. The more horrific you make the sacrifices Slaanesh demands in return for the transformation, the more in character it will be. Like many things Slaaneshy, you'll need to be careful to retain a PG rating. Just remember that it is far better to hint then to state in graphic detail.

[Tfcdogbert]

Origination Idea: Sisters of Battle.

Overview Not as stupid as it sounds, The sisters are the premier fighting force of the Church, Quite often associated with the Inquisition, they can be linked into creating a different force.

Points in Detail We look at the marines and their position to the inquisition, the two are not easy bedfellows, the inquisition can call upon the marines but they do not neccesarily have to answere, so we get this situation where the best warriors of the imperium are not neccesarily at the inquisitions disposal, this understandably is to grate upon the =I=, they wish to have something in which they may have more control over a force like the marines.

 

So we get to the nub of the matter, the intake of the sisters comes from a suitable set of people if my memory serves correctly, the orphans that are entrusted to the church, similar form to the marineal selection in that the best of all of them go away into this new training program, now female young children grow in much the same way male young children do with similar hormones and so forth, this is until their early teens when they start Estrogen production, which stops muscle mass and growth and height etc, a chemical that would inhibit oestrogen production would stop the female development of their bodies and let them continue (in theory with supplements such as steroids and testosterone) to grow at the rate of men.

 

Now with that the only issue might be the issue of the geneseed, while i am no expert in the fluff of how space marines grow and so forth i believe that an inquisitor with sufficient resources behind him can overcome such obstacles, i will need some help on that subject however since im not so much of an expert in what the 19 geneseeds are and how they work, im sure the problems can well be overcome, the imperium will be full of clever people, especially with inquisition poking that would not hesitate to grab the string of genetics and give it a substantial pull.

 

Perhaps even work the faith of the sororatis into a more substantial change to the geneseed, how they were originally created was from the Emperor if i remember, the closest thing we have to the emperor is the living saints, touched as they are by the purest of forces.

[Melissia]

I was asked to comment on Tfcdogbert's post in regards ot the Sisters...

 

 

1: It is likely that the details of what is happening would need to be kept secret from those outside of the program. The Sisters are quite close-minded about genetic alterations and mutation, and they have fought rogue Inquisitors before.

 

2: The Sisters are trained first in the Schola Progenium for most of their childhood. This is the same place as stormtroopers and commissars. It is only after graduating from the Schola Progenium that the applicant is sent to a Sororitas Convent, and even then likely only after it is determined that this is the best place for her, as opposed to the commissariat or the stormtroopers. An Inquisitor could very well pull recruits from there, but doing so might earn the ire of the Commissariat or Sororitas for removing potential graduates and so it must be done with care and tact.

 

3: The Adeptus Mechanicus has the ability to manipulate genetics, and in fact do so on a regular basis in the creation of the organic part of the bodies of their mechanical servants (for example, they can manipulate an embryo's genetics so that it only produces a brain and skull, nothing more). All the Inquisitor would need to do is obtain favors from the AdMech, enough to convince them to do this experiment and do it right. This is actually probably the easiest hurdle to overcome, given that the AdMech has a stockpile of gene-seed anyway.

[Lion in the Stars]

Hrm. Lots of good idea sparks so far, now just need a discussion of what makes something visually female versus visually male (and I don't mean adding breasts to a Space Marine!).

 

(Ferrata, if you don't want this discussion here, feel free to nuke/relocate as necessary, please PM me if you do)

 

I don't have a good set of photos to play from, but I'll try to describe things verbally, then chase down some photos to back up points.

 

1. Women are more slender-looking than men. Their arms and legs are not as bulky as a man's, and their shoulders aren't as wide. Look at the difference between the greaves of a Marine, versus the greaves of a Sister. They're both in power armor, but the sister's legs and arms are much more slender than a Marines (arms less noticeable). A woman that works out all the time like a professional bodybuilder still doesn't have the sheer bulk that I do (When I did work out a lot I was 6', 190lbs, and wore a 48 suitcoat with a 30" waist). Although her arms may be just as big as mine, my chest and shoulders are built differently, and are both wider and thicker. This is true whether you supress estrogen production or not.

 

2. Women have a narrower waist and wider hips, while the narrowest point on a man's body is at the hips. (Note that this will change with estrogen suppression). A model that doesn't have a narrower waist doesn't look female. How many young ladies have you seen that didn't really have hips (yes, they are out there)? Do they look like girls (or young boys), or like women? The stereotypical female form is just as wide at the widest part of the ribcage as it is at the hips (anyone remember the 'perfect' measurements of 36-24-36? For comparison, my numbers when I was in Marine shape were 40-30).

 

Armor: Due to the different physiological structure of male and female skeletons, any armor designed for women needs to be different. While this is less important for power armor than for worn armor, it still exists. Women cannot carry anywhere near as much weight on their shoulders as men, but they can actually carry more weight on their hips than a man. Also, from a practical standpoint, you would not want individually-armored breasts, as the area between them would turn into a shot trap, making it more likely to get shot through the heart! This results in a different armor shape for the chest: Usually a slightly triangular shape when seen from the side, with a slight ridge running vertically looking from the front. Add a narrowed waist and wider hips, and all of a sudden, your model actually looks like a female, Space Marine big, in power armor. After all, if all you wanted was a girl in PA, you'd play Sororitas!

[Grimdarkness]

Origination Idea: Desperate times call for desperate measures.

Overview: In his attempt to rebuild hid legion corax gathered huge numbers of potentially viabel (note that full grown men at the time could be truned into marines) marines lets say a hand full where young woman. In his grief/maddnes corax implants them with the gene seed he played with.

Points in detail: Unlike there male counter parts the group of females take to the altered gene seed well and show no sing of mutation disparate to find out why corax resined his order to cull these mistakes and proceeds with a battery of test. Haggard and tired of the administration of the emperors peace corax leaves his legion and his pet project behind. The females trained to the full extant of the "brother" marines seize the window open to them and flea.

[Melissia]

Origination Idea: Gene-seed experiment side effect

Overview: A new chapter is formed using slightly modified gene-seed, but the modifications have the side-effect of switching the gender of the young boys as their journey to become Space Marines commences.

Points: Those that were recruited younger would have the most complete transformations, while those that ended up being recruited later in their childhood would be more masculine. Also note, provide a reason for the modifications-- perhaps they were descended from the blood angels, and the modifications were made to cure the black rage... perhaps they succeeded, but with side effects-- or perhaps they failed, but still had the side effects. Also note that unless it is stated that their male genitilia is removed or otherwise goes away due to the mutation, then they would actually all be hermaphrodites, feminine forms with both genitilia.

 

Origination Idea: Travel through the Warp

Overview: The shield protecting the ship falters during a trip through the warp, and some fo the chapter's marines are strangely turned female along with other general bits of chaos.

Points: One could keep the chapter together, or one could split it on gender lines. Keeping it together would be fine, but they'd have to set up two types of recruitment depending on wether or not the marine giving the gene-seed is male or female. Separating them would create two chapters, sibling chapters that could lead to some good fluff if you do it carefully. They would also need to worry about inquisitorial review due to the somewhat obvious taint of chaos.

[Atomic Top Hat]

Might as well throw my army background out there.

 

Origination Idea: Angry Daemons.

Overview: After angering a greater daemon, a warband of marines have had their geneseeds taken from them and implanted into a group of lower class servant girls.

Points: This is probably cheating as far as backgrounds go, but Chaos is aloud to do that sort of thing. The reason why daemon decided to create a mob of space marines out of uninspiring maids (of course, something like an army of all-female guardmen would work just as well,) is left vague. It could be because it wanted to cause an uprising or have taken a liking to them or just because it's chaos. The fate of the original space marines is a matter of question. The daemon might of just killed or could of turned them into stunted deformed versions of their former selves, force to serve their new masters until the end of time.

[Grey Mage]

Origionation Idea: The "Golden Woman".

Overveiw: When the emperor was intumbed a woman who glowed brightly was at his side along with the primarchs. She was infact the progenator of a line of femal space marines who like the Grey Knights serve a specialised purpose.

Points: What exactly is their secret purpose? How do they keep it hidden? These are very imprortant... you could say they were psychic nulls, created to fight threats like the eldar, or in the emperors far reaching sight, the tyranids. Some good reasons for their secrecy must be established aswell.... NOT "well they fell under a rock/warpstorm/blackhole/your mum for 10,000 years you see". Perhaps they are the real "Adeptus Custodes" and thats why they were so rarely deployed and are so rarely seen.

[Yogi]

Originator Idea: Zygotes re-keyed.

 

 

Overview: Some artifact/STC find, plus a genius medico; re-keys zygotes. This is done for the purpose of creating better warriors and/or decreasing production time of making a marine. Plan is sucessful a chapter loyalist female marines are created, however catastrophy occurs to artifact and genius medico (inquistion or enemy attack etc) so chapter is unique among space marines.

 

To me this is the best way to represent loyalist female marines.

 

EDIT: If you wondering how you get someone to believe you can rekey zygotes in your IA instead of trying to explain how, simply show how it works similar to how sci fi shows make believe ships can go faster then light. Usually atributed to something like warp engines but never explained. So for example rekeyed zygotes were possible due to the brillant Dr Vexmar's employment of the Omicron Resequencer, discovered in a damaged STC's memory banks, etc. See easy, who can argue when you have created a device that does what they want to believe impossible.

[Sister Kyrie]

I'm a fan of the missing primarch idea, already mentioned above. Here is an idea of how/why they got removed from Imperial history w/o going Chaos. It obviously still needs some work, but I thought it makes a good starting point. Maybe someone else can help me refine it.

 

At the start of the Great Crusade, the Emperor takes all his marines off to wherever to get started. The legion in question gets a call for help from their homeworld/one of the planets in their home system. They ask for permission to go help. Emperor says no, cuz the planet is a lost cause. Unable to accept this, the primarch takes her legion and prepares to head off. The Emperor tells her that if she leaves, he'll disown her and make it as if she and her legion never existed. Primarch figures she can patch things up w/"Dad" after she gets back and leaves. Some disaster occurs while they're away to prevent them from returning to the Crusade fleets, things never get resolved, and the Emperor follows through on his threat. Voila, missing legion, no fall to Chaos to hide, but still fits the "forshadowing" of the Horus Heresy in that they rebelled. How'd some of them end up in the 41st millenium? The Warp does strange things...

 

I also second Dr. Thunder on these points:

In my mind, a Female Marine would have less physical strength then her male counterpart, and would also be shorter. She would also be quicker, have a higher pain threshold, longer lifespan, and superior ability to track multiple threats simultaneously. None of these differences would be great enough to require stat changes, but the presence of a trade-off in abilities makes it feel more in character with the fluff.