You know..... I think I’ll just hit up the creation tables on 1D4chan, I’ve held off making my own chapter because (I’m a filthy heretic) lots of them are really cringe/gay and I’m not about that.
Any tips guys?
Tfw when I rolled an Ultima Founding Ultramarine successor.
I'll sperg out a bit. So my dudes are all Blood Ravens, but I have this reoccurring weird mental image of an Irish / Celtic themed recruitment world, with enough connections that I could do MyDudes of anything I want connected to it.
So in my mind there's an Eyes of the Emperor known to the locals as "The Old Man of the Mountain" based on the planet, keeping an eye on the local region of space, which leads to a shield host worth of Custodes tending to come and go from the world at times. There's an AdMech expeditionary force that's stranded in the area because of the great rift and has made a minor forge on one of the subcontinents or moons out of necessity. There's a not-quite-Knight-house, more like Freeblades-of-a-feather -- something like the Three Musketeers, only Imperial Knights -- Hanging out using the AdMech for repairs and supplies. That kind of thing -- they're all stuck there because of the rift, so they're having to put up with each other in closer quarters than normal, but it's somehow working. Imperial Soup: The Solar System. But the Blood Ravens, being stuck on the wrong side of the Great Rift, are stuck recruiting from this world occasionally, too. Which has a distinctly different culture than normal Blood Raven worlds, namely, they're significantly more laid back. Think: Perfect Kill Team Members, Not so Perfect Line Members.
This leads to cliques being created in the Blood Raven lines from recruits from this world, as shown by them painting parts of their armor an Emerald Green instead of Red, to set themselves apart from the other Blood Ravens, as well as including Celtic crosses and patterns, like how the Grey Knights / Thousand Sons sometimes use glowing runes on their armor -- that, only Celtic patterns. Maybe these recruits are almost all, if not 100%, Primaris Marines, as well, this all occurring after the great rift and the Custodes brought the Ultima Founding equipment to the Blood Ravens.
Basically, I couldn't decide how to base my miniatures, so I started some on top of astrogranite, some on top of the mud, and realized if I combined the two in patches on newer bases then it'd look intentional.

Which made me think of doing the same thing, only with miniatures... So a Blood Ravens force where a handful of them have increasing parts of Green, Celtic themed armor... Exploring how an existing chapter "naturally" comes to see a new chapter grow out from them was interesting to me.
However, If I ever did anything of the above, I'd probably just start a fresh Vanguard Space Marine army instead of trying a "gradient" thing, though.
Failing that, I did have an amusing idea of a western / cowboy themed Space Marine chapter, because holy shit every other niche has already been done, but somehow not that. Lots of good suggestions on a /tg/ thread when I brought it up, but finding it now would be hard as hell.
Edit:
For later remembering:
- Custodes force based around Eyes of the Emperor, "Old Man on the Mountain." Model him with a barbed spear and a hood?
- AdMech expeditionary force, creating small forge on a moon cause they're stuck ("Styges VIII / Mars / etc, but not")
- Not-Quite-Knight-House, "Freeblades-of-a-feather," Imperial Knight force based on something like the Three Musketeers?
- Space Marine chapter "budding" into a second chapter
- Cowboy themed Space Marine chapter.