a game mode where one team controls the scarab, and the other team will have rocket launchers and warthogs
also the ability to control the scarab in the single player mode.
a mulitplayer mode where you can be spartans, elites, burtes and the flood, and have a multiplayer game mode specific to playing them all.
underwater level, with new spartan armour in single player, maybe some new type of underwater covenant.
a villain mode where you can control the elites, or grunts in single player mode, and a friend can control the master chief online, but he wont know which grunt or elite you are.