Since the XBox Live APIs are going to let you have a mixture of local and network players in a given game session, the idea is to have multiple XBoxes (at least two) taking part. One team of four would be on one xbox, while the opposing team is on the other.
Team Collaboration
Ever since I played Battlefield 1942, I've been enamored with multiplayer collaboration. The fact that it is possible for a well organized team in that game to mount a calculated assault and totally own the enemy is very exciting. Of course, notice I said "it is possible" ... I've never actually been in an online game where there was any level of organization :-PThis is why I'd like to develop something where the collaboration must occur with people physically on the same box ... kinda like when the power rangers form up the megazord and they're all in the same "room" (does each one control a limb or something? :-P ). Or perhaps how everyone has a role on the bridge of the enterprise.
Obviously, none of this gibberish that I've written has much depth ... these are all just ideas that I'd love to explore. I don't expect it to have mass appeal, because you'd have to have a fairly large group to play it (8 at least) physically in the same location. But that's the great thing about XNA ... one can create things that don't have all of these expectations of ROI.