Whenever I mine, I just see cave systems and ores and just abandoned mines and the occasional fossil. So here are my ideas.
GIANT CAVE SYSTEMS - So usually cave systems are narrow and the like. However there are things a giant cave system has.
STALAGMITES AND STALACTITES - Stalagmites and stalactites look like ice spikes except that they are made of stone and occasionally, ores.
Mining Hub - These are rare structures which have villager miners and blacksmith. They look like abandoned mines except that its not actually abandoned and there are glass, torches, and villagers in it. Also there are underground houses.
The Mantle - There will be tons of lava and magma right above the beddrock. However, more ores will spawn here, but it's so dangerous.
Anyways those are some of my ideas.
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