My crack-theory:
Spoilers
We will be going to Garlemald but we will also be going to future Eorzea.
Like in the future we hopefully prevented, a calamity will destroy civilization.
La Noscea, the Black Shroud, and Thanalan will all be big maps, their individual ARR parts put together, with the city states in ruins. Maybe we will see some of Coerthas but Ishgard will be lost.
Mor Dhona might survive (and be our new hub city or end game hub) due to the Crystal Tower.
Back in current times, Garlemald will get like 3 areas.