I really can't see them letting any of it being 720p. They've put a lot of work into improving how people perceive the X1 (in regards to power). Letting that slip with Halo 5 of all things would be really dumb. I'll happily take a bet on it though You can pop my bet making cherry.
900p/60fps seems like a safe bet for the SP though. It could be higher, but assuming they also want to introduce some graphical improvements, it seems like the most likely progression from where the beta is. That or 1328x1080 (what Halo 2: Ani was), but that's pretty much the same as 900p.
They can do 1080p with 60 fps, but it will sacrifice some graphic, if they want to do more they need time to release on 2016, even if they put 1000 worker that's still to hard to create house engine that can run code to close to metal like what Naughty dog does, and for what i can see they need to focus first with multiplayer first and second with graphic, i think we will see some graphic improvement with Halo 6, or game that will be release on 2016-2017 onward. But it's still to hard to judge for now, but still my concern is not with the hardware but with human resource and from what recent multiplayer fiasco with MCC. It will be great if they can deliver promising multiplayer aspect with some graphic improvement, but MCC told us differently so we will wait and see.