One thing that has not been done yet with FFXIV

[SturmChurro]

New cities become ghost towns because everyone's busy AFKing in Limsa and Ul'dah. Where we also can't use mounts.

I don't really know how lively the expansion cities have been, but there being so many of them, and the early cities being accessible is also probably a factor. Not sure how RPers justify it.

An underground zone can be fun, it could be interesting. Like what was mentioned previously: the underdark. It has been done, or is being done, recently with Baldur's Gate 3, and it showcases colorful environs, and there is the possibility of interesting creatures as well. There's also Skyrim with Blackreach as an example, etc..