In addition to overworld min/max heights, I'd like to see a "can see sky" config. I imagine it as a percentage where 0.0 means it always spawns underground and 1.0 means it always spawns on the surface. To match current behavior, 0.5 could be the default, where it doesn't care one way or the other.

This way if you want them hard to find (low height values), 0.0 would help prevent them from spawning in open ravines. If you want them easily spotted on the surface, 1.0 would prevent them from spawning inside hills.