Crimson Skies. I know that Microsoft doesn't own the license but they could license it and make a new game. That was an amazing dogfighting game (a genre there aren't many of these days) with an actually good story for the time. Dark Void sort of tried to do something similar (at least in terms of tone and having dogfighting, though mostly it was a third person shooter) and that game was janky as hell but I still enjoyed it, despite being blatantly and obviously unfinished with the most abrupt ending of any full retail game I can remember.
But a new Crimson Skies with next gen graphics and a slick Uncharted style presentation would be incredible. Plus the multiplayer was great in that game so that too!