Halo 4 Thoughts

This afternoon I read this article in which some professional gamers speculate and offer suggestions on how Halo 4, still 2 or 3 years away, should work, look and feel. I enjoyed the entire 3-page write-up, but there was one idea in there that really grabbed me. Maybe it's already been done, and I just haven't heard about it, but when I read the following:

"online co-op will once again play a large role in a Halo game. Halo 4 should feature twice the number of online players as the third game, expanding the final count to eight. But here's the kicker, those eight players can be split up into teams during campaign missions. That means that one team can play as the heroic UNSC soldiers while the other four players try and stop them as the Covenant"

it nearly blew my mind! Imagine playing a campaign level in online Co-op mode, with some of your buddies in the game alongside you (and hooked up via headset), but instead of having to get past the usual AI bad guys, you've got actual humans trying to stop you! I'm not even sure it would work, because AI characters are generally designed to be short-lived (and who'd want to play a role of that sort?) but maybe the game designers could figure a way around that. For example, if you were on the team that was subbing for the AI's, then possibly you'd re-spawn a certain number of times and then switch to a different AI character, further along in the map? The whole thing has my head spinning, thinking of both the opportunities and challenges involved. I wonder if anyone's ever tried it?