I'd like to see them do more with Halo. It is their biggest IP , but we only get a game every 3 years or so.
I think a core Halo shooter every 3-4 years is fine. But, what about a halo dog series? There is tons of more that could be used as the basis for such.
Maybe they could do a remake of Halo 3, or CE. Both games are monumentally important parts of gaming history. I'd live a reason to give them another $60 for those games again.