In a recent topic about realism in equipment design, it became clear that we all have different notions of what is acceptable, silly in good way, silly in a bad way, what is cool, what is just dumb, etc.
The great thing about the hobby is that even if you're not creating a homebrew sub-faction, you are still free to modify the universe to your will and preferences (as long as you follow the rules on the gaming side of the hobby.)
So I was wondering what additions or changes to the established 40k universe do you consider canon in your head?
EDIT 1: I'm especially interested in cool and creative additions that both adds to the established universe while also "fixing" aspects you're not too fond of, without just ignoring those aspects.
An example that I recently added to my headcanon is the function of Space Marine Ancients, or their banners specifically.
I've always thought standard bearers were silly. Partly in a good way as it feeds into the medieval aspects of space marine chapters and banners can look pretty cool. On the other hand, I find it hard to justify in my head why they would bring these supposedly priceless relics into battle only for them to be torn apart by a few bullets. Do the banners really provide such a boost to morale that it becomes worth it? To me this works for the more zealous and impractical chapters, but in general it feel a bit weird.
So in my mind, space marine (and maybe other faction's) banners are mini shield generators of some sort, providing additional protection to a command squad or a key objective that needs to be held at all cost. These shield generators are small enough to be carried by a marine, but big enough to put on a pole, then decorated over time with banners and such, thereby attaining symbolic value as well.
EDIT 2: In hindsight, I realize now that this topic might just turn into another place to vent frustrations, which, for what it's worth, was not my intention. I feel we have enough of those around here so hopefully we can avoid too much of that.
EDIT 3: And please, comment and build on other peoples ideas and thoughts, but pretty please, don't derail the topic into a battleground.