Honestly, the clues are aimed at Garlemald, but this could be a bit of a red herring, as the main subject of the expansion could be elsewhere. We might actually finish off Garlemald for the most part before the next expansion, or in the early chapters of it. As to locations of interest, Sharlayan is possible, there is also Several lost empires mentioned during heavensward, we might get more Dragon content as well, plus we have a few worlds to learn about, might want to prevent their eventual rejoinings, and we should deal with the Void World.

Honestly we have a lot of work in the future, as to what they focus on, hard to tell.