This is why I've gravitated towards Blood Ravens. They remind me of myself.

Not sure about my family history (there's a lot of missing pieces), we both like shiny objects, and make really bad tactical decisions.

I mean, if you are using Primaris as the main force of your army, then you can have them be from the Heresy/near Heresy era, as Cawl took a bunch of potential recruits from that time period to begin to experiment with his enhancements. So you have a chapter's worth of Primaris Marines who are all from the same planet during the Heresy-era- maybe they volunteered like the Ultramarines' neophyte stock or they were feral worlders like the Wolves. Now however, they are a newly formed Ultima Founding Chapter, deeded to protect a world/system that they do not know. Their motivation is to protect, but they don't know this new place they are in; how to they integrate with the system authorities, how do they recruit, do they try to integrate their old world's culture onto this new place?

If you have an older firstborn member or two who has crossed the Rubicon and been transferred to this new chapter to give them advice on how to interact, that may be an interesting hook and possible way to make their motivation more interesting. If you have a particular First Founding/Successor chapter you want to be descended from, make your Firstborn from that chapter.

So I have Planned, built or started my share of Space Marine armies over the years:

2nd Edition- Space Wolves - I liked the Viking Aspect- Sold

3rd Edition- Dark Angels- Liked the Ravenwing and robes- Sold

3rd Edition- Black Templars- Starter box art work got me plus Emperor's Champion- Sold

4th Edition to 6th Edition- Crimson Fists- The Old Rogue Trader art- Never Finished

6th Edition- 7th Edition Raptors- 10th Company Raptors- Sneaky scout army- Sold

6th Edition- Deathwatch- Special Forces. - Sold got some back from friend that bought them

7th Edition- Ultramarines- the blue is nice- Never started

8th Edition- Imperial Fists- I like Dorn never finished model and added to another army

8th Edition- Raptors Phobos- Scouts no longer troops- Sold

9th Edition- Mortifiers- I like the split scheme added to another another army

9th Edition- Raptors- Phobos no longer line- Sold

10th Edition- Black Templars- Melee- never started

10th Edition- Blood Angels- Red melee- Never started

10th Edition- Imperial Fists (again) rolled into Exorcists

10th Edition- Exorcists- Serve the inquisition

10th Edition- Deathwatch- Serve the Inquisition- if I get back to them

To me it is the little things about the army and why I wanted to build or paint them. My focus has been on Inquisition aligned units, more specifically Ordo Malleus supporting Grey Knights and Exorcists and Ordo Hereticus supporting Ecclesiarchy. But all this ties to the Planetary System my gaming group fights over. 

I suggest doing them as an DIY Chapter and an Allied unit for your other Imperial army and go from here. 

I even started building up the lore for them.