On 2 disk idea:
I think...
Disk 1:Campaign, Firefight, Firefight Forge(my idea) and Theatre.
Disk 2:Matchmaking, Multiplayer, Forge, Theatre
Every vehicle in any Halo game(even Scarab/Frigate) available for Forging. A.I. spawns is an awesome idea, and so is O.D.S.T. drop pods.