I was thinking maybe some mushrooms and different ores, probably some other things that you would find underground (dripleaf, azalea bush, amethyst shards, axolotl bucket, sculk catalyst, etc.)

I figure they wouldn't have most things found only above ground, or maybe they have them but are more expensive because they can get them through trading but they don't have nearly as many as the aboveground villagers

I have been trying to work on a mod that generates underground villages (I'm likely gonna call them "Ancient Villages" or "Deep Villages" or something similar) in the normal Deep Dark (not the Ancient Cities because that would be multiple types of chaos) with some of the houses built into the walls and with mushroom farms and other underground type things

of course :)