I've seen tombs (chosen teams, 8 players/team)
Anything else?
Chosen 4 vs 4 teams would be nice
To be able to "challenge" another team would be nice
random and chosen 12 vs 12 would be insane!

1 vs 1 duels would be fun (Healing monk problem)
2 vs 2 would be quite fun as well
...
Why limit to only 2 type of pvp?