Xoroth - the homeworld of the dreadsteeds (and possibly the nathrezim and tothrezim).
K'aresh - the original homeworld of the ethereals before they evacuated to wander the Twisting Nether.
Argus - the homeworld of the eredar race of magic-wielders and their exiled brethren called the draenei.
Out of these three planets, you could have at least two amazing expansions. Argus is the most obvious one, draenic revenge and beating up eredars is always fun, although this is likely the last destination in WoW expansions. K'aresh would be my first choice just because you are fighting against the void and it would likely make ethereals as a playable race. And plus almost everything about the ethereals is pretty interesting.
And the least likely choice, Xoroth, home of dreadsteeds, and the Tothrezim, who are arguably the most interesting race of creatures that have not really been fleshed out in the game. They live in the twisting nether, cousins of the nathrezim, and they are servants of Sargeras as well. This would be a great expansion to set the stage for the climatic battle against Sargeras. Plus Xoroth is not very well described so they can basically create anything they want for this.
Which of the three planets would you be most interested in seeing an expansion for?