Mountains need a rare volcano, caves need a lava biome with flowing lakes and rivers of magma. Give those sulfurous waters a reason to exist. The big plus is ambience, I think, though others might have other takeawys based on what was added with it, and possible mechanics added.
Some foraged items could be added, maybe increased gems (which is actually a thing) and ore. Could be dangerous to be there too long, though as it'd be hotter and riskier. Could be some events like random pyroclastic flows, eruptions (spew small boulders across the map, possibly including ore and gems). Add in a new fantastical beastie or two, maybe more likely to find slimes or boreworms (and decrease elsewhere?)
(Note: found a very short mention of it in another thread, like just the words "volcanic biome." Thought it might use some more discussion.)