Getting back to Halo Reach, one thing they could do would be to maintain the open world concept of fighting across the entire planet and having to choose which missions/battles to pursue. Also, it would be awesome to have win/loss scenarios such that winning opens up different paths than losing (and losing an objective would NOT be a game over/re-start). In essence, a highly branched mission/objective structure that plays out despite winning or losing a scenario. It would also have to be time progressive where different things open or close based on how far into the overall battle the team is. A true evolving battlefield that reacts to the players successes or failures would be a great twist. After all, we already know how it ends...
This would make the game highly replayable too. You'd probably have to replay it a couple of times just to play through most of the missions.