I would honestly be really happy to have an expansion that went back and expanded on previous areas; there's so much untrodden territory that's just kinda gonna get left to rot if they don't. I wasn't a 1.0 player, but I'm told there are still several areas that never made it back into the game, that might be a good place to start.
And literally any Endwalker area would be a great place to start, since cramming an entire nation into ONE SINGLE ZONE is insane, and we barely spent any time in any of the nations we visited, so they could easily expand any of them. Both Ala Mhigo and Doma would also be contenders, since the way Stormblood split the story meant they both got short shrift.
If they'd finally acknowledge that time is passing in-game, the First could also be a great place for a new story, as people start re-exploring and settling the areas that were previously inaccessible due to the flood.
And I can't be the only one who would absolutely lose my mind if they chose to revist The World Unsundered. Have the WoL get knocked on the head by the first boss of the expansion and spend the rest of the .0 MSQ roleplaying their way through a dream about some of Azem's adventures. That would have made a much better 'vacation' expansion than Dawntrail, since it could just as easily be used as a way to lead in to the next major arc, OR a 'filler' that wouldn't effect the stakes in the present at all.