Firefight - It would be fun as hell, as well as work in the Halo lore. Make it more like Horde, so it gets more difficult with each wave, instead of playing the same waves again.
Invasion - Whilst I didn't like Reach that much, one of the few things I liked about the game was Invasion. It was creative, intense and it's just fun.
Dominion - Unlike many, I LOVED Dominion and I was sad when they took it off the playlists. It's not as good as Invasion, but it's still fun.
Multiple HUGE Forge maps - I liked how Reach had a huge map for forging, I also liked how Halo 4 had multiple Forge maps. Well, there was Forge Island but most of that is water. They should have terrain editor as well, and bigger natural Forge pieces. What with this being new hardware, this could be a possibility.