So imagine this: You're exploring a vast cave system. You're lost and don't know your way back up. You're low on torches and food, and all of your weapons and tools are inching closer and closer towards their breaking points. You have no choice but to keep moving forward. Just when you think it's over, you see something. An opening at the end of the long hall of stone you seemed to be trapped within. But it isn't the surface. You step through, and to your surprise, it's a massive cavern, and within it is a fully formed village.
Now obviously, cave villages would be EXTREMELY rare so as to keep the core mining experience intact. Maybe not "stronghold in a superflat world" rare, but pretty rare all the same. The buildings would probably be built using a combination of various types of stone as well as possibly the materials available in the surface biome the village is in. Most things would probably be the same as surface vilages, although maybe with a few tweaks (more torches to better light the caverns in which the sun never reaches, a higher likelihood of iron golems due to the fact that caves are more dangerous alongside the fact that iron is more readily available underground, stuff like that).
That's pretty much all I've got. I hope y'all like the idea!