I loved Caves in H&H, they were mysterious, a little creepy, and great for hiding stuff in (I hid a whole town in one, as did many folks).
Unfortunately Haven style caves were an integrated part of map creation, so they can't be added now with out regenerating the whole map. OR CAN THEY???
I propose that caves be sprinkled over the map using the Node system currently used for mines and wells. The Cave would be found with the woo-woo wand alongside wells, but would be considerably rarer then wells, maybe 1:10 or 1:100 ratio. With the wand equipped the cave entrance is visible and can be entered if you posses the 'Spelunking' skill which requires medium to high Mines&Mountains. No alteration or construction is required so the Cave remains concealed for anyone not similarly prospecting. Inside the cave is darkness and has darkness humor drain on top of what ever the existing drain of the above area was. Foragables only appear if a torch or light source (lantern?) is equipped as well, so no lame client side evasion.
Now what is the point?
Whole new Biome!
Unique, rare cave foragables, great for WITCHCRAFT!
Places to hide stashes of treasure, silver, gold etc etc
Elite monster spawn zones with commensurate new elite drops