Ok so I was thinking after seeing the set up camp mod and the cave biome mod that it should be possible to spawn holes on a normal map like geothermal vent which, if you find one and build a ladder you can make an event where a little pocket map spawns (like set up camp). You can then set up an expedition down like forming a caravan and go down where you enter a cave like biome with cave type stuff , new monsters, mushrooms , gems etc. :)