There are quite a few little hollows, or passages, suggested in caves and similar that don’t do anything currently in-game (see an example of one <here>).
I think it would be nice if we could explore them. They could lead to some kind of mini-instance. Nothing too serious or complicated, perhaps a random little cave network, containing, a concentration of a particular resource, with some enemies therein, perhaps even an NPC thrall/merchant to save with varying options to purchase items from them or have them return to your base, etc. once you have.
Maybe they spawn a procedurally generated tunnel to explore. I don’t know; there are a lot of options. I think something could be done with them.