I found a semi anarchy survival server called CaveMC and the whole overworld is underground. There is no surface at all so you live in caves forever and still get biomes resources and structures.
It feels like enhanced vanilla and it is meant to be brutal. You have to be smart to survive and get ahead.
Season 2 is out with a fresh start and new additions. They are also opening a new place called the Wasteland Dimension with its own gear to fight for big mega structures with secrets and a unique currency for a player driven economy. There is also a new spawn and a custom generated map for that dimension.
