As requested, here's a list of everything suggested for cave biomes:

Nether cave sections/Nether "geodes"

• generates in the overworld
• features natural Nether resources (blocks, plants)
• spawns Nether portal ruins (if possible)
• generates at a very low Y level

Undersea Coral caves (living and dead)

• features coral blocks and plants, kelp, general underwater stuff
• generates in ocean biomes, caves are fully flooded

Slime cave sections/Slime "geodes"

• generates only in slime chunks at slime spawn levels
• either fully made of slime blocks or have them spotted here and there

Bee cave sections/Bee "geodes"

• generates with bee nests, honey, honeycomb blocks
• possibly feature some plants/flowers

Nether Quartz geodes

• generates in the Nether
• speaks for itself, but also kinda ties in with Nether cave sections/Nether "geodes"

Have geodes consist of various stone types

• for example, pure andesite/diorite/granite geodes laced with gems