I'll put my money up for Pandarens, just because I want to play as one. No real reason, I jsut do. Except, Blizzard said they originally the had plans to place them in somewhere, and I honestly couldn't see a better place. (Just so long as they are at least NPCs, I'll be satisfied.)
Though, I also may place a few chips on the Draenei, just because they are ugly and hate the horde. If Blizzard was trying to give Horde a pretty face, they may believe they are balancing it out by giving the Alliance an ugly one.
If there are any kinks in the story though, Blizzard will change it like they have everything else. Like moving Draenei's home or creating a motivation for the Pandarens to fight. They can add whatever they want.