We talk about haven being potentially eternal, but our current mining system has a pretty restrictive amount of mining "space" for serious players. It might be worth considering a semi-frequent appearence of temporary underground cave entrances leading to "pocket dimensions" that are a mineable cave layer as a renewable source of minerals. On the surface this can introduce new materials for newbies like copper in a tin zone. Could be used as a location for interesting or different monster spawning.
Seems perfectly in line with current world mechanics when you consider dungeons.