For PVP a team mode is definitely needed, party on party action! For these events you could have special PVP Arena game types that allow a higher number of players to join (maybe 10 or 12) so they can team up and compete. A variety of game types should be able to be initiated by talking with NPC's such as a direct team vs. team deathmatch, or a time attack boss rush (for Slayers, Champions, and the like only). The winning team could be rewarded with a very nice loot drop.
If going the route of being able to host different game types (such as the aforementioned PVP arena maps) perhaps you could also allow hosting of 'Market Maps', also with a higher max player count (this time up to 20), and it only contains space with stalls where players can go set up their own shops and go afk. This would also help keep gold as something useful, since market transactions would all be in gold, and the easiest way to sell your unwanted stuff. Players could have a message board inside their shop screen so they could list specific items or trades they were looking for as well, allowing other players to leave their player name and a brief message so the shop owner can contact them later.
Of course this could all work very well into the new Battle.net, you could have three different tabs where games are filtered into Questing, PvP, and Market types. Then whether the player wants to progress, compete, or look for an item, they can just click the tab and be shown only games pertinent to what they want to do. The different tabs could be linked to different chat rooms as well. Keep up the good work, I'm sure Diablo 3 will be awesome!