Utilizing the same system that compiles Missions for Game-Specific or Cross-Game Playlists, I think there should be an option to allow players to make their own playlists, compiling Missions from across one or several Halo games, in any order the player desires.
Now, I’m sure you could just combat this by saying “well, just play whatever levels in the order you want.” But that doesn’t offer the same freedom as Playlists. In Playlists, beating one mission immediately starts loading the next Mission in the Playlist. Otherwise, it starts loading the next mission in the Campaign, and you need to wait for that to finish, quit, then load into the mission you want next.
I feel like Custom Playlists would be a fun little addition while not taking too much work to do. Heck, even if it was a case where you could only make/do one Custom Playlist at a time, and there was no saving feature or anything. I’d be just fine with that (if not a little befuddled).