These are a few of the things I think should be in halo2.

1) More than on team in multiplayer.
2) Custom maps, ones players coudl design.
3) More than 16 players per game but only possiable on bigger maps.
4) Bots, like in perfect dark.
5) different characters, like one that is taller and better aim, or one that is shorter and faster than the normal guy. Or a guy that can jump a little higher, the possiabilities are endless.

Add your ideas.