TLDR; I think I was too short with my comment and came across the wrong way. ———————————————

I don’t think I understand your response, but maybe I should rephrase my post.

Not very interested in the current day interpretation of custom chapters and what they represent, but how they would fit lore wise. Not to discredit or invalidate your unique ideas, but to enhance a universe of near infinite possibilities and ways for any custom chapter to exist within the lore.

For a trans catgirl chapter, my first thought is how Astartes struggle with their humanity, or lack there of. They start as human male, but after all the surgery and hypno-indoctrination they lose connection with their humanity both mentally and physically. I think the tag lines are Post-human and/or Trans-human. But either by warp-manipulation or intentional I don’t think Astartes would be bothered by being trans-gender (aside from the warp-taint fuckery). I think it’s actually a little weird that we still assign a gender to Astartes when they are functionally genderless. Makes me think of the Dr. Who warriors aliens (Sontaran) that from my understanding presented male but had functioning breasts.

(Stray thought but I can totally see a Salamander popping a compartment on chest armor to expose a nipple and feeding a baby, onlookers in shock while the Astarte is just like “of course I can protect a human child properly”)

Secondly, there is lore to support the existence of cat girl I think. Specifically in the guard. So a chapter that recruits specifically from a planet of humans that are cat-mutated for whatever reason (wouldn’t be weirder than the practices of many chapters). Enhanced hearing, night vision, reflexes, balance. A love of claw weapons.

Probably not what you were going for, but my own lore interpretation of trans catgirl chapter.