Honestly, I think Phil needs to see if he can find a way to get Bungie back in the fold. I feel like they both kind of need each other, and would profit immensely from getting back together. Bungie did it's best work while they had MS oversight, and really the only thing that made them want to split off (far as I know) was the desire to do something beyond Halo. That's obviously not an issue anymore. On the flip side, I'm sure there are still plenty of people there who care a lot about Halo, and there could be some collaboration/consultation between them and 343 on possible directions and things MS could do with their flagship franchise moving forward, or even the occasional passing of the baton...where maybe Bungie makes another Halo all of their own, and 343 are freed up to take a bit of a creative break where they can make something entirely new.
And while I'm not the biggest fan of much of what Bungie has done lately, there would just be something special and awesome about it if MS came out at E3 in some future year down the road, and a Halo trailer starts playing, with all the usual bells and whistles, and then at the end, all of a sudden you see the Bungie logo there, and that's how they announce that they're back where they belong. I mean come on. That's a "you win E3" moment.
So ya….me, if I was Phil, I'd be pushing to make that happen pretty fucking hard.