For as long as most of you will remember, Bungie and Microsoft were like *this*. Bungie made Halo, Microsoft published it. But these days, Bungie fly solo, and while they've got a deal in place with Microsoft for the short-term, in the longer term, their options are open. Bungie's Brian Jarrard:
No, we're not [tied into a specific platform for new IP]. We haven't gotten that far yet to be totally honest...In theory we do have every means to go out and find the right ecosystem, the right partners, to align with. So who knows what the future will bring us.
We know what it'll bring you. Regardless of the quality of that first non-Halo game, it'll bring you money. Bungie's Back - Part Two [GI.biz]