The major loose ends right now I think are:
- Dalmasca... but they'd have to make the Ivalice Raids and Bozja mandatory in order to build an expansion around that, and I don't see that happening;
- The Thirteenth, which would require Warring Triad, the ShB Role Quests and subsequent Void Quests, which would still not be as arduous as Bozja, at least, imo;
- Meracydia has been namedropped a lot (in the Coils of Bahamut, Warring Triad, most recently in the MSQ), and we have the cooperation of the Dragons now too, so it's not out of the question... but whatever story they set there would essentially be entirely new;
Then again they could do...
- Something completely out of nowhere, like ShB was;