I mean, obviously a new Halo has got to be in the works. There is not Xbox without Halo... I doubt there will be a Halo available on release of the console but if there were it would definitely be a boon to Microsoft so who knows, maybe they'll push one out as fast as they can. Similarly, Gears of War is an IP that perhaps is getting a little bit stale and old these days and they just came out with a new one but they're probably working on a next Gen one as we speak.
Now for what I hope beyond all other things is a Half Life 3 exclusive (not counting PC) release for the One. Holy smokes, can you imagine how amazing that would be? Valve, please do this for me! Please announce this at E3 Microsoft, oh please oh please.