It's a bad idea in general, there's nothing appealing in pvp in d3.
D3 is just not meant to be a PvP game.
They would have to implement way too much stuff (matchmaking, balancing etc. etc.).
And I doubt there's a huge request for pvp, so it wouldn't be smart.
You could just play WoW (very similiar when it comes to classes DH-Hunter, Monk-Monk, Barb-Warrior, WD-Warlock etc.).
I just don't see any point in implementing PvP in D3. I'd rather have more PvE content.