Personally, I really enjoy playing alongside other CPUs in Campaigns. It gives me that “I’m apart of something” feeling. In the Halo 4 Campaign I really hope that we will have plenty of Marines and ODSTs to fight along side with. It would also be nice if they increased the amount of CPUs in one area during campaign, so maybe we can be fighting hundreds of Enemies along side squads of UNSC forces. It would definetely give the players a large-scale warfare feel, which many of us would like.
Depends on how many are on the UNSC Infinity and when it shows up. That said, I still don’t see why Forerunners couldn’t make good AI partners.
Personally, I’d like the Reclaimer trilogy to follow the same trajectory that the Halo trilogy followed: I’d like to play the beginning of the story in relative isolation, struggling against overwhelming odds, and in later games I’d like the UNSC to become a real presence so that, like you, I once again feel as though I’m part of a larger conflict. I think that that sense of isolation in Combat Evolved was what made the eventual introduction of the full UNSC so thrilling; it wouldn’t work to simply begin the game in that way, attempting to artificially make the player feel “part of something”.