I think it would be awesome to have a Hal Lgcy game, where 343 re-does all the great and important levels frm all the Halo games and puts them all together as kind of a "best of"
They could go in chronological order so you play through the best moments of Reach first, then follow MC from Reach to Halo CE then jump back and forth between ODST and MC's adventures as they are happening at the same time. New cut scenes could show some of the stuff in between that connect it all together.
Multiplayer would (ideally be outsourced t Bunge haha) have all the best maps and playlists from all the games, and in Forge you could create Firefight levels and Objectives versus Covenant enemies. Like after creating a level you could place different enemy types and create conditions for victory, such as kill them all, or reach a certain place and shut down the reactor, get the object to an evac, etc.
Legendary edition of the game would come with flaming helmet, and legendary 360 bundle would come with blue flaming helmet and armor. Armor effects such as Grunt Bday. Storm, and hearts would be back for your Spartans, and some armor pieces unlock with credits, while others unlock through commendations or achievements. This way you are not just automatically unlocking a bunch of stuff you would never want and can work toward the appearance of your armor the way you want it to turn out.
What levels, weapons, features, etc. would you want to see in this game ?