Well as for the surface part I don't really have many ideas besides making a more ocean focused space, maybe small islands with a large ocean as the main map feature, even though that could become a bit empty... Maybe custom caves/cave biomes, or even a giant cave focused map a kin maybe to ark's aberration map, with a mostly barren surface and a variety of underground biomes. I have no clue how possible these could be but they're the only ideas I have as of now.