Sheldras Moontree was the head druid trainer (or so it looked like) in The Park of Stormwind. When Deathwing took a little vengeance upon the trees and happy courtyards, he managed to escape with a few others out to the docks, where he currently resides. When you approach him on a druid, he says an oddly forecasting statement:
"There will be a time soon where the call of the Emerald Dream will take hold in you. You will hibernate for many years and walk amongst the purest of forms. It is there and then that your ultimate training will take place."
Well. What does that mean? First off, Stormrage was only a setback. Obviously we are going to the Emerald Dream at some point, but in no way can it be as peaceful as our Lunar Arborist suggests - because there is no such thing as class-specific content any more, remember?
What I think is going to happen is that some time before the expansion is launched but in a world event, there will be a little class specific content. Druids will be called together and will enter the Emerald Dream. But all will not be as was hoped - and Old God, or one of their puppets, resurfaces there and the entire fight begins. The druids have to open a massive portal to Moonglade (all sorts of potential here), where a huge staging ground is built and (just like in Ulduar) the horde and the alliance aren't teaming up this time.
So the expansion Starts - using a heavy amount of phased content - when the invasion of the emerald nightmare/dream begins. Unlike Outland, where bases and outposts are already made (Honor Hold must've taken months to build), this one's entirely fresh - any quest hub will be the barest of places and entirely phased. Tacking in monthly world events that are seemingly in the works, this hypothetical expansion could be quite epic indeed.