As a suggestion, you could probably check out SMTIV, which paints demons and lucifer in a more sympathetic light. The idea of one side fighting for order and the other side fighting against that order because it restrains individuality and freedom would be a good way of keeping both sides grey rather than strictly black and white.

I'm happy to discuss this if you guys are still interested in making this game. I'm really hopeful you'll go through with it!