Playing Arx Fatalis made me realize that I don't really see too many subterranean fantasy settings! I kinda wanna fill in a gap by making my own cozy place for all sorts of magical antics. Have it focused more on lone settlements finding each other and anchoring down, scarce numbers of people moving in between them due to the limiting size of the tunnels.
Or maybe it opens up the deeper you go, and entire nations are just a few layers deeper. Of course, this means longer falls and more treacherous conditions to get there, so they don't just have everyone trying to rush down and congregate places.
Or maybe I just love the gay little clomp of my boots on stones, and a setting all about wandering about and surveying an ancient world where you suddenly spawned in tickles my fancy >w>