My personal favorite would be (most likely not):
A new action game like God of War. I heard the former Team Ninja leader and his bandits are working on a new action game. Figures cross.
Also, with the success of a exploring/action/adv/shooter like Uncharted 2, I think Microsoft would like to have a game like that in their arsenal. The question is who will make such a game for them since they don't have a 1st party studio that makes these kind of games.