By the power of truth, I, while living, have conquered the universe.
Add in a few sections of the campaign where you play as Zuka, the Sangheili hunting John down over the Ring. That would be badass. Also, a section playing as Rtas when he boards the infinite Succor and fights the Flood would be awesome.
A bonus section where you play a mission based off the Mona Lisa story from Halo Evolutions would be great too.
Bring in Firefight, some areas from the campaign (like the second mission) would be absolutely amazing because it's such a big playable area. Invasion on the Truth and Reconciliation would be so fun for MM.
[Edited on 12.28.2010 9:16 AM PST]