During War Within, Xal’atath will raise the long-lost (super secret) islands surrounding Quel’thelas from the bottom of the ocean. Also, playable naga.
Well, the obvious answer is that the Quel’thalas front is only part of the expansion, or even not really that significant to the expansion proper as opposed to the storyline. (e.g., Cata started by trashing Stormwind and Orgrimmar, but didn’t spend a significant amount of time there in the expansion itself).
I can imagine them breaking up the Ghostlands quite a bit to have a scourge, void, and restored section. Then there should be an entire region for the Amani, as well as a newly built Isle of Quel’Danas that would likely have multiple levels, and for everything else the can just create new islands