First off, we have all heard the idea to limit the PvE play to somewhere around four to five players. I just want to say that I would be very disappointed if this number was not able to be overcome in PvP. At least eight players, and if possible, even more.
Areana Style PvP I feel is needed. Even a matchmaking system if possible. This system could work where you can talk to an NPC who then asks you what type of PvP you would like to enter, be it free for all, one vs one, two vs two, etc. Then based on a type of PvP ranking (maybe like the averge if you are in a party), a fair match is found for you. I know Battlenet is not really setup to work this way, so maybe make a way for you to create a party within the battlenet lobby, and search for a match from there. The same idea still works.
Note: I feel there should be certain level breakpoints in this matchmaking, such as level 20 and under, 50 and under, 99 and under or something of the sort. This may only be worth it however if low level dueling will be an option in Diablo III, becasue I do not know enough about the mechanics of the game, I am not sure this will even be feasible.
Also, make sure there is a way to have strick team based PvP where you can party with friends, and get in matches against other teams. Team based PvP was awsome in Diablo II, but it was slightly difficult to ever arrange in a public setting. It usually ended up in 6v2 or something of the sort if it wasnt a private game.
Also, make sure there is a way to have organized clans with tags. In this way, when a party is full of all clan members, there could be a special option for a clan battle in which case there could be a clan ladder also.
I would also like to see more options than just one setting for PvP. Like a random set of eight arenas or so that you are randomly placed in once a match is found.
As we know, Diablo PvP is really fast paced, which is the reason I think so many people are die hard fans, cause the PvP is really unlike any other. Because of how fast it is though, I feel each PvP match should be rally scored, and the first team or player with a certain number of kills would win. In this way, when a player dies, they could be allowed to respawn in a safe location (their base) where the enemy is unable to enter and camp. Then the battle would continue. If it was last player or team standing, I feel you would spend more time searching for matches than actually playing becasue of how fast those matches could go.
One last thing, once a match is found, I think there should be like a 30 second counter, and as soon as that counter is up, the walls of your base or something similar drop down, and you are permitted to leave to fight the enemy or enemies. This would make sure teams didnt have the chance to get an unfair location advantage.