Title says it all. Yes, we have just had Garrosh in SoO, but if we waited an adequate length (I've no idea how many expansions that should be; maybe 1-2) who do you think should be de-throned from their position? For myself, I'd go for Sylvanas or Gallywix. The former is steadily becoming the Lich Queen, and if we had a lot of focus on her during an expansion, then hopefully I'd like to find out what the hell is lurking under Tirisfal.
As for Gallywix, he's a greedy asshole who cares more for business/money than he does for the welfare of his race. The sooner his throat is slit and someone takes over (like Donais), I'd be happy. The idea that Goblins are just a one-note race that care about money and engineering is annoying. Give them some character development.
Note: I didn't add Jaina as she isn't a faction leader.
But what is your logical choice? Please explain why so I can read all your interesting theories!