Well, i love making huge mines in Stonehearth but i think that just digging in the rock is too easy and boring.
So i think that adding those things would be cool for an underground adventure :
1- Caves to explore with chests, monsters and rare ore like gold.
2- Goblin tunnels with wooden structures,chests and lot of goblins that will get out of their tunnels to steal your goods and attack your settlement.
3- Lava can be found on deep mines with some kind of fire monsters and dragons. (Maybe in the Lavasmith update).
That’s all but i hope something like that will appear in the game later to change a boring mine into an awesome adventurous gameplay