Here is my idea. Normally, you can only build a mine right on top of the metal deposits. But I think you should be able to build a mine anywhere, but that isn't to say there will be metal. You would simply get rocks dropped on the floor. And mines should all be connected, like if you built two mines next to each other, you could mine the gap in between, as long as it doesn't exceed the maximum amount of tiles in a mine. Also, mines should need supports. You can build 1 x 1 supports, 2 x 1 supports, 3 x 1 supports, and 2 x 2 supports to hold up the roof in caverns. They all take one plank, and they all stop a 5 x 5 area from collapsing. Collapses can occur anywhere in places that aren't supported, even if it hasn't been mines out yet, though it isn't as likely. One collapse causes collapses to be more likely in that area. And mining causes collapses to be 3%-ish in areas not supported, and being under one means death.
And mines should be more like caves, with stalactites and and natural caverns. There should also be pure deposits of metal nuggets (except steel, bronze, and iron of course) anywhere in any type of mine, but in limited quantity, to get that "YEEEHAW I STRUCK GOLD" feel.
And possibly suffocating. Like, you should have to bring a canary in and if it dies, get your but outa there. Distance from the enterance increases likelyhood of death.
Ok I'm done. And I love you Delamore, don't hurt me. I love you...