1. Keep Firefight, but add more maps and characters and different modes for firefight. Like perhaps flood only enemies or no drones or something. Something to make Firefight different for each player.
2. Down grade the Spartan Laser. How about three shots, and it takes five seconds to charge. The laser should be used on vehicles ONLY.
3. Make the campaign better. More epic fights, and the campaign should be AT LEAST 15 hours long.
4. More skulls, like 20 Gold and five silver. New skulls could be things like enemies respawn until you pass their spawn locations, and a skull that makes melee attacks have no damage.
5. More multiplayer maps. Twenty new ones and ten remakes would suffice. Let's see Headlong, The Pit, Guardian, Sanctuary, Blood Gulch, Last Resort, High Ground, Ascension, Beaver Creek and Epitaph remade.
6. More armor. Keep all the current armor and add tons more, and make another armor that takes the vidmasters to unlock.
7. Make creative achievements, like how about 100 plasma pistol kills in multiplayer? The vidmaster challenge: Classic should be a regular achievement. A cool achievement would be earn a killionaire in any matchmaking game.
8. Keep Halo "The Same", and I mean that like no weapon classes PERIOD. No UNSC knife PERIOD. No perks PERIOD. No run button PERIOD. This is Halo, not COD.
9. Update Forge. To difficult to go into detail, read this thread.
10. Fix Action Sack. Make it give EXP, and remove team rockets and team stickies. And run for cover, they all suck. Lets see more grifball, team fiesta and ninja assault.
I think this would make Halo: Reach awesomesauce.