Ive thought in something easy to implement and cool as a late game stuff.
Like in heroes of might and magic there could be scatered around the map, maybe by found by mining or in some caves, or fishing inside a bottle,or in some npc villages or whatever, some pieces of a map. That map could lead to a place in level 5 of the mines, only accesible by digging with a superb treasure. Like an incredible gear or something like this.