My solution for halo is not to add new game-play features, but take what put them on top on the first place and make those parts really shine.
I played halo over other FPS's because of it's not realistic anything. It was my future alien FPS. How fast you could move and jump, the over the top multi-player vehicles, The crazy weapons all with a different niche in combat. These are the things I remember and constantly wish to experience every time I boot up a halo title.
I don't want an overhaul, or even a revolution, but rather just new content, new weapons, new maps, new vehicles, all while maintaining the core mechanics that made halo great in the first place.