I actually have been working on a system that does just that but it hasn't been implemented yet. That's a good idea with the succubus/demoness girl. I actually do have a skin for that that looks fairly close to that. I have some more basic stuff that still needs done but once I've addressed that I'll look into hooking up the enslavement and corruption system.
The game already contains the ability to play as different characters, it's just not implemented, so it wouldn't be hard to add in the ability to swap between them. This is a cool idea and the concept of swapping to someone more humanoid is nice. I'll give it some thought and see when I can get to it.