As long as Bungie doesn't have the rights to Halo, they can't use anything from the halo series. So, if they wanted to add some small detail, like a race (the elites, grunts, hunters, etc.) they have to have permission to use the material. If they have the rights to Halo, they have more freedom on what they can put in their games. Considering Bungie references their previous games alot, it would disable them from doing so without the rights to Halo.
Actually now that I think about it, BECAUSE Bungie does that, it pretty much means that Microsoft has to ask Bungie just to make a Halo game. This would be because most of the Halo series has many things based on previous Bungie titles, which Bungie still has rights to. In my opinion, Microsoft won't be able to create a Halo game as good as Bungie because most of the ideals in the Halo series are Bungie's.
Microsoft will not give up its rights to Halo.
/thread
Unless they're new game (games if you count the new Halo CE) does bad on the market or it loses them money.
[Edited on 07.12.2011 1:32 PM PDT]