First off, I know that bungie said that halo: reach was the last hurrah, but i have an idea to keep the series going, while still making it different. Second of all, if this is in the wrong forum, oops.
Going back to the end of halo 3, the elites leave and are all buddies with us. However, i refuse to believe all of the brutes were at earth or the ark. Bungie could make a game from the perspective of the returning elites finding sanghelios under attack by brute forces. This way we could create another world, bring back arbiter (although not necessarily as the playable character) but have a different type of conflict than just, "yay smash covenant". It would give an indepth look at the elite home world and their social, religious, and political infrastructure.
Idk just my idea