A very well written campaign story that satisfactorily resolves the story arc of H5 without ruining the legacy of you-know-who, which I have no idea how they're going to pull off at this point. (Cause it feels like they pretty much already did.)
A much longer campaign, which is actually played as the Chief.
The return of split screen.
For the campaign soundtrack to maintain at least the quality of H5's. (In other words, please keep Jinnouchi.)
More PVE gametypes, like a PVE-only Warzone, or the return of Firefight or Spartan Ops. On a related note to that, co-op campaign or PVE matchmaking.
More launch playlists possibly including one or more with Spartan abilities disabled (which will require maps being designed that don't require SA's to move around them) for those who prefer classic play. With all the debate and frustration among the fanbase about classic vs. modern (COD-like) Halo gameplay, I fail to understand why we can't have both.
More casual playlists/more emphasis on casual, noob-friendly multiplayer - which can coexist alongside the ranked, high-intensity, eSport-oriented multiplayer.
Would not mind seeing the REQ system return, but I don't feel it should be used for armor. Give us a Reach style credit system for that. So people can actually get the armor they want with hard work/grinding, and not leave it up to random chance.
I guess that's about it for me.