The only way it could be done without a total rewrite of the game would be to allow players to create 'cave' portals into a small single room 'dungeon' which could then be expanded by the player using an interface to a version of the dev tools used for creating the caves (ok not quite that simple but that's the gist).

Problems abound - the tools would likely have to be (re)coded from scratch, an entirely new UI and build system would need to be created, and at the end of the day player's would complain about the various differences and limitations from the present system (not to mention comparisons with other games).

It would require as much work, if not more, than a half dozen new biomes. All for something comparatively few players would likely use.