The bottomless pit aspect of the chasm and bottomless pit(harhar) would have to go in favor of a simple pit/canyon that extends down from the surface to some level below, maybe with sedimentary walls to wall it off from the chasms it passes through, unless it's hard-coded to always stop at the first. It could then have the old caves sticking into the walls in various places, where semi-mega and megabeasts could dwell as well as in normal caves.
The underground rivers just plain need to come back. They could be essentially the same, but instead be a surface river that flows into the ground at some point, or start underground in regions with high drainage and rainfall, where there isn't a single water source like in the old, but many dozens or hundreds of 1-tile sources long the length, and continues down into the caverns, until it hits an underground lake/sea. Geysers should also appear where water meets heated rock, spraying the water upward, maybe to the surface.
ugh... an illustration will help my brain, how about you?
Also and unmentioned, udg. rivers should be able to continue while IN the caverns.
In a last-ditch effort to save the thread from myself, I'm probably going to remake my first interesting fort, Bloodwines. They were brutally slaughtered at the hands of goblins and my lack of udnerstanding of how to properly defend them (they had walls, though) I'm going to try to redo it all. in the same biome, with a brook in a dense forest, no mountains, and try the same basic concept with some necessary overhauls. The outdoor area will be bigger, have higher walls and have towers, have a gate (I didn't know about drawbridges or floodgates, so i was helpless except for my military) and the fortress itself won't be so cluttered and difficult.