I've always wanted to a play a game where you are a serial killer where the objective is to simply kill and avoid capture (like don't leave evidence behind, etc.). perhaps there could be a deeper purpose, some evil master plan (like the movie Seven for example). Or maybe you simply start out killing prostitutes and work your way up to more important targets (thus making it harder to get away with the murders).
There have been several games over the years where you are the "bad guys" but nothing to this extent (that I'm aware of). I would love to explore the evil side of humanity a bit more, see what kind of emotions it brings out. haha