Couldn't have told you this until I had read through a lot of these other requests, but what if in addition to the return of a proper Theater mode, the was also a Photo mode for PvE modes? Like can be seen in a lot of third person single player games this gen (Assassin's Creed, Spider-Man, etc.)
Could be a cool thing to build into the new engine from the get-go, and a great way to get some monstrously cool shots of Chief in the campaign as well as our own Spartans in Firefight (Firefight coming back being my "common" Infinite request).
I think this could be a sleeper hit feature. Obviously not the sort of thing that will draw accolades upon announcement, but I think a lot of players would use it, myself included.