Get rid of the reach engine(no class set ups and no perks unless they are placed in the maps)
Bring halo back to the way it was in halo 2.
Where you had a adventurous campaign, a real tight structured multiplayer, but also had the glitches in it for the private custom match "fun" play, and i personally wasnt the biggest fan of firefight....maybe if you would add more gametypes(obsticle course, warthog races, bomb defusions.etc).
Also take a page from MW3 right now with the community playlist where they put the fun private gametypes into matchmaking. I would love to re-live the good days of halo with "tower of power", "hide and seek", and any other fun gametypes the players would come up with.
Just a thought. Halo 2 was the funnest game in my opinion to play. Would love to experience that again since they shut halo 2 down for xbox live.