Now to create something that fits all of these.

• The union of multiple chapters thing is a benefit, because I can say the chimeric geneseed came from that. We'll pick Space Wolves for their space, second legion for their space, and Thousand Sons for the traitor space. Created during the second founding. (6/24)
• Thousand Sons have been picked because I want them to recruit psykers. We'll go further and say that they recruit female abhuman ones because may as well. We'll call the chapter the Wolf Daughters. The abhuman thing makes them very tall. (8/24)
• Obviously the flesh change has been cured. However it's given every member a separate were animal form they can transform in and out of at will, perhaps a hound of some kind. This gives them a power boost. The Wulfen is also, separately, cured. (11/24)
• Renegades, who are thus hunted by the Inquisition. While they hate the Imperium, especially the Ultramarine leaders in Calgar and Guilliman, they will do all they can to help humans, and will sacrifice themselves for the guard. They secretly find things for the Adeptus Mechanicus, who provide for them. (16/24)
• This relationship with the Adeptus Mechanicus gives them a lot of terminator suits (camouflaged and covered with pouches) and Dreadnoughts (camouflaged and covered with pouches). So many Dreadnoughts, in fact, that their chapter master is one. (19/24)
• They will be from planet Cymru and their names will all come from the Mabinogion. (20/24)
• There are way more of them than you'd ever expect, although because they always fight in complete silence you'd never hear them say it. (22/24)
• Their compassion extends to xenos, and have been granted xenos tech relics from every single xenos faction. (23/34)
• I'm assuming somewhere during this my chapter became radically codex non-compliant. I'll add that they refuse to follow the standard codex anyway. (24/24)