Isn't it already annualized? I mean, they've done some different stuff with the franchise compared to Call of Duty, but we've had a Halo every year for a while now. Odds are we'll be getting Halo 5 this year and Halo 2 the next. Maybe Halo 6 after that, if not, we could see MS giving the game to somebody else for another different styled game. I'd love to see them work with Epic's People Can Fly dev to make a Shadow Complex style Halo game, which I think would be amazing. Sequel to ODST would be nice as well. Maybe make make a vehicle based combat game as well. Either Star Fox-esque combat game like the space combat levels in Halo Reach, or something Twisted Metal style.
MS could treat Halo a little like Mario in a way when you think about it.