Dragon Isles seems to be a recurrent and popular request. Come on, who doesn't love dragons in their fantasy games? A dragonsworn class would be sooo awesome: mail wearing, 3 specs... Perfect.
The other side of Azeroth is also a clear winner. There absolutely must be something, anything west of Kalimdor/east of EK. It would also be the perfect opportunity to bring MoP 2.0: lighter theme after so much darkness and edginess, new class, completely new and let's hope, huge unnified continent. Who would be its inhabitants? Why was it hidden for so long? Countless opportunities for greatness!