This was suggested awhile back, and I by no means claim credit for it.
1) Free for all
2) 50+ players
3) Arena-style maps
4) Location/kill based gold & experience
Discuss.
Teamwork would still play a large role, but those teammates would have to turn on eachother eventually. Without a multiplier system, there's no real incentive for winning. However, maybe someday some kind of betting system could be put in place for spectators and dead players to keep it interesting, though it would have to be done carefully to prevent blatant abuse.
I think I would rather have the single player tournament system. But of course with my gear :PThat would be neat. After thinking about it, I don't think there's any real way to prevent betting abuse, so no point in leaving it in the OP.