One strong element that the current games have is that your character can slide into Insanity or Corruption. I hope that Black Crusade keeps this, with some modifications of course.
Insanity can stay more-or-less the same. Chaos followers are crazy, but if you become too raving mad to function, I imagine your "friends" would use you as raw material for a daemonhost. A little tweaking of the rules is all I think they would need to do.
Corruption is a different matter entirely. For those of you with some grey hairs, you may have been lucky enough to play the Chaos Warband system from the Realm of Chaos books. Truly wonderful stuff. The lore is that while many Chaos Champions aspire to daemonic immortality, the majority die in battle or are transformed into chaos spawn. Your champion would earn "Favor Points" in the service of the dark powers, which would result in Gifts of the Gods or Mutations. Along the way, there were several ways your champion could be "spawned," While only a very high roll on the "Chaos Reward Table" would transform you into a Daemon Prince (and even then, only if you had become powerful enough). A modification of this would be perfect for the RPG. When you accumulate 100 points (Favor, Glory, whatever), you make a final check to see if your character is rewarded with Daemonhood or cursed with Spawnhood. I know that a lot of people will rage against this idea, crying that if they lose their character, it's no reward. If the system is set up properly, earning these points will not be easy & they will benefit the character in some way (lure of Chaos, right?). Also, a true roleplayer will be quite satisfied that his/her beloved character has acheived their ultimate goal. You can bet that the new Prince would be proudly converted to lead a aemonic Legion on my tabletop. Just wanted to share my thoughts. Thanks & good gaming!