One thing that has not been done yet with FFXIV

[Stanazolol]

I wouldn't call wow's Deepholm exactly a dark depressing slog. And it was underground.

And most D&D games do the Underdark in a way that isn't dark and depressing to some degree, just usually dark..ish.

Not saying it can't be done, but most of the time underground zones are a chore to get through rather than something you look forward to imo.