At the moment, you don;t get any net worth benefit from caves, so making an underground civilisation doesn’t really work. My suggestion is how about a way to have worked internal walls. There could be a few types - wood panels via the carpenter, brickwork via the mason, clay tiles via the potter etc.
You would construct those, and then designate walls in a mine. Your hearthlings would go and place it, making pretty walls instead of “raw rock” walls. Any rooms fully dressed like this would then contribute to net worth, so your underground civilization could flourish, as well as extra “credit” for doors, lamps, fittings and so on.
Plus, it would make mine tunnels etc look a lot nicer. There could even be different patterns of brick, wood panelling, etc.
Not too complicated a change, but something pretty cool. What do you think? Is this somehting the devs would consider, @sdee?