I actually like the idea of trying to keep the soul of someone else to get what you truly desire. The one and only downside to that is that our characters will literally be trying to keep away from each other most of the time, especially if they get their own soul back. xD So it would be a very solo-driven roleplay, in my mind. But we could probably work something around that. I doubt that Angels would get involved, and Demons would probably be mocking the players the whole time. 

Maybe we could consider adding more rules to the game. Such as, either everyone has a pair (someone that won't have their soul), or everyone has to stay together for a certain period of time? I'm just trying to think of a way to get more character interaction in, unless you don't want it. In which case, please ignore my ramblings. xD