I'm actually bringing this back from the old forums again, but I think it's an awesome idea. It would require huge maps and creative mapmaking, but there's really no shortage of creative mappers out there =)
it's called capture the tower
basically, there are several towers strewn throughout the map. red team starts on one side, blue on the other, and each controls the tower they spawn at. there will be a flag at a base near this original spawn point, and in order to capture other towers, a player must bring this flag inside another tower (it goes up top like in Mario when it's claimed), and now other players on that team can spawn there. Anyone killed will spawn from the most recently captured tower.
Now the competition comes in because opposing teams must capture all but one of the towers. To capture a tower that's already claimed, you must bring your flag to that tower and sit there for 10 seconds.
You only have one free flag at any given time (once you claim a tower a new flag respawns back at your original base)
it can probably be improved, but that's what i remember...