Druid or Paladin or Rogue
It would be great if the expansion would add a more open world and be more sandboxy without the quests once you reach Inferno (they TALKED about it recently!).
And within that structure a shape shifter or stealth class would be awesome.
They mentioned this open world and sandbox several times in the interviews, so cross your fingers...
It would lure a LOT of players back in I guess.