Friendly Sangheili. Having Elites to accompany you in Halo 2 and 3 was awesome. Why did they take that out for Halo 4?!?
A linear plot, but one that isn't repetitive. I'd like a variety of different mission styles. Halo 3, Reach and 4 have that variety, but it kind of repeats in a cycle: "find this", "activate that machine", "destroy that", "find this", "activate that machine", "destroy that", and so on.
A better splatter system. In Halo: Reach, splattering was too easy, in Halo 4 it's too hard.
The return of elephants or mammoths for multiplayer, probably as bases.