Ahh...lets see:
An American Football game that would allow the user to play past season's in a career mode ala OOTP.
A comprehensive Political simulation
(I'm dreaming big-time now)...where you start out as a grass-roots activist or what not and plot out your political career from there to state/federal government, make decisions, interact, and just 'play' politics. Sooo many variables, this would be a hell of an online game with lots of players to represent different countries of the world, interacting like a real geo-political simulation.