Some structures repurposed for usage in Deep Dark caves (using a deepslate palette) would be cool. In particular:

• Ocean Ruins
• Outposts
• Jungle Pyramids?

Instead of mobs, they could potentially spawn with a warden pre-summoned.

Spawning in caves is not an easy thing to accomplish with structures and also runs the risk of performance issue as I have to scan for the cave.

Ancient cities are massive so they have a good chance of interrupting a cave system to be discoverable. Smaller stuff like outposts, ruins, or temples would be next to impossible to find unless I manually scan and stick them into caves and even then, they will still be very hard to find.

The Underground Bastion works because it is a high loot, large structure that I have explorer maps that locate it from wandering traders/land ruins. The three structures you proposed are too small and low loot that it isn’t worth making an explorer map to point to them.

Also, I am not a fan of outposts underground. That won’t look good in my opinion and cannot be filled with pillagers as they cannot survive sculk

Flip the outpost upside down and name all the pillagers dinnerbone