I agree with the 'more HFS-like things underground in areas'. I like the antmen nest idea, And the queen would be a... Gi-Ant! You could also have networks of naked mole dogs (imagine prairie dogs), molemen, and other things... and when you hit their nest, they swarm out like a siege at full speed! And you could have different types of antmen nests, including leaf cutters (HUGE nests, with LARGE wide open chambers seperate from eachother, including a 'fungus chamber' where a patch of underground 'soil' has plump helmuts permanently growing on it, and a 'dump chamber' where long-decayed antmen remains could be found. And having various types, from ant eggs (edible meat) to soldiers (big and nasty like ogres). All just like real Attine ants), army ants (bigger, nastier antmen, possibly carrying weapons and armor and such, and coming in huge swarms), the usual benign ants (smaller chambers of regular antmen, though very abundant in comparison to the others), and Bullet Antmen (similar to bullet ants conceptually, not a large number of ants per colony, but they are themselves like semimegabeasts in difficulty).
And the having slimes and stuff in deep caverns isn't as far fetched as people think... look up 'gold mine bacteria' and 'radiotrophic fungi'.
And yeah, digging creatures would be a wonderful idea! No longer can you hole up with a bridge over a moat permanently while goblins mill about. Now you must FACE THEIR CHALLENGE OR DIE TRYING MUHUHUHAHAHAHAAAA
ANd yeah, geysers and hot springs are a great idea. You may never know what you will get when you tap an underground water feature! Some could be blatantly dangerous! I have spelunked in mines in Idaho that were abandoned because they hit a hot spring and a bunch of people died of steam burns.
And I agree with the ability to find long-dead civilizations' ruins. One could excavate items and put them to use or put them on display. And the items can be considered legendary but always be worn and torn, like X*clay bucket*X sorts of things, or X*ancestral sword*X. Still worth a lot, though. And it would be rare, few and far between.
I must ask: how does one get this painter program people are using?