Honestly though, I would love more detail on aliens in BC. I want to hire Kroot mercs and have Hrud spies. I want a Dark Eldar beastmaster with a group of rare and exotic carnivores I can feed people who defy me to. The rules for alien allies are limited because loyalists should not be having any contact with aliens. BC is clearly the place to include them
Absolutely. There were a bunch of Chaos-corrupted xeno species glimpsed in the Gaunt's Ghosts novel Traitor General ; BC seems like the perfect way to explore that aspect of the 40Kverse.