Before I say anything, I would like to let you know all the inspiration I got was looking out a window.
We all know caves are a natural thing and so why not add it to the game?
It leaves tons of opportunities such as hidden traps, very easy mining access, and a quest for loot! Plus: It’s an awesome place for goblins to spawn in. Also, with caves could come mine shafts and such (but just for the aesthetics of course) And maybe just so it isn’t as easy, what parts of the cave/shaft you haven’t been in is fogged off so you won’t be able to make “safe choices”. This could add a very intriguing amount of curiosity and suspense to the game which many of us would surely enjoy. Plus a little more to explore!