About Geodes and Crystals Caves: you'll find them near the bottom of the first list under "Geodes"...I expanded out the potential size (this is fantasy afterall)...so they could end up as cave-size. I will add the link to crystal caves in there. Thx Angry Lawyer.
The original Hell biome entry is still in there, just renamed. Once you start talking about a portal to hell, you are no longer talking about the underground.
Instead of trying to remake the current HFS to fit out fantasies...real diversity would be found by developing new types of HFS. A good example was the trapped fire spirit found under "Urist Rogers in the 4th Century!!!".
Rock Bursts are going in, with link, and being lumped with earthquakes (already there) and probably getting melded with unstable rock. I'm still trying to condense similar ideas into categories or entries.
The permanent rock idea is a bit unrealistic. I agree the way to simulate that is with the rock hardness suggestion. Some rocks should just require a legendary miner using a masterwork steel pick to mine.
"Urist McNoobminer cancels dig: unable to scratch surface"
To get the "permanent rock" feel, small clusters of these hard rocks could be scattered through all the z-levels. Late game you can clear them out, but early you'd have to dig around at least one.
Speaking of cave-ins korora - I have not been following the two cave-in threads much. I plan to read them and dovetail what we can, but does any one have an unbiased summary?
Edit: For legibility and correctness: feel free to suggest alterations in the groupings and point out spelling, punctuations and grammatical errors. The list has grown so much (and is so hard to see with all the tags) that I am likely to miss some really good way to improve it. I will always be trying to chop the big "general list" into smaller pieces to avoid the tldr mentality.
edit: I want to advertise a non-UD, but related game mechanic thread:
A Consolidated Soil / Rock system (and Rubble)