Literally will never happen and there is absolutely no chance whatsoever of it happening unless Sony was to buy Microsoft.
The Halo franchise is owned by Microsoft. Why would Microsoft give their powerhouse shooter franchise to their rivals to benefit financially from. It simply wouldn't be good business sense.
Not saying that it wouldn't be cool, because it would.
EDIT: And Microsoft is that greedy, but in an American Dream kind of way. However, I'd like to remind you guys that all the Microsoft executives could easily retire and leave us with poorer leadership as they spend the rest of their days in massive mansions on their own private islands. So it's not really right to give them crap for being greedy.