There should be an "[X] mob can spawn naturally in this structure" tag.

Biomes and Mobs have tags galore to control how they spawn, but Structures don't have anything of the sort.  If you want a certain mob to spawn naturally inside a specific structure for datapacks or mods without using spawners, you're out of luck.

So having a structure tag that says, "This mob can spawn here, with or without direct sky access" would be really nice.