I'd like a world generation game. This game would gen a world ( in a similar manner to DF ) and you can pause the world at any time. It would have a detailed history, with races and civs popping up, having their own lives, journeys and sagas independent of you.
The player would be able to be one of two sides: One side would be demons, and the other angels. If playing on the side of demons, you can corrupt the creatures and races and civs of the world, to an extent. The more specific your corruption, the more effective. Trying to evil-ize the entire world at once would have an extremely negligible effect, if at all. But, if you were to focus all your power and efforts to corrupting ( both the body and soul of ) a king, it wouldn't be all that difficult, and his madness, mutation and moral decay would then influence all those near and underneath him.
Same goes for angels, who instead, try to purify the races of the world, and inspire men and women to acts of bravery, wisdom and compassion.
In the middle would be randomly generated beasts and forgotten monsters, completely neutral in the battle of good and evil, but can be used to serve either end. A player on the side of demons can corrupt these beasts, drive them mad and have them besiege the poor races of the world, as if they were in an old fashioned giant monster movie. A player on the side of angels can alternatively have these beasts become protectors of the civilisations.
One side would win once the entire world has become either pure good or evil, an extremely long and arduous task.
This sounds goddamn amazing. However, I'd like for there to be a multitude of moral paradigms there could be selected, ie good/evil, order/chaos, bacon/necktie, etc.
Also, Galactic Civilizations 2, with a very significant improvement: the map is now a cube, not a square.