I keep thinking about this mod a lot but didn't find a compelling reason yet to use it. Wondering if it would be feasible to have some resources only spawn underground? I'm currently playing with BZ mods like Tin, Lead, Diamonds/Graphite and especially Diamonds and Graphite might be perfect for only spawning underground

This is on my todo, but needs some good balancing throughout different mod setups.
For vanilla, only uranium, calcite (Vulcanus) and maybe raw holmium are good candidates to spawn only underground.
For BZ, I think graphite is needed too early in the game to be only underground. Maybe diamonds and titan.
Since even different BZ setups significantly change recipes, this is a hard balancing act. Not to think about PY, SE, Bobs and other overhauls.

But in addition to that, another compelling reason is needed than just resource generation changes. Like some production chains that are themed underground. But I'm uncreative as a piece of shit.