Current mining is crap, hopefully the many suggestions are implemented soon to give better mining.
One addition that has probably been suggested before is your mine caving in when you have too much open space.
A simple rule like the old school DF 7 tiles with no roof support = cave in.
If you want to limit the amount of dirt that can be held up by supports, maybe have some sort of formula where empty tiles increase pressure, and supports reduce pressure by an amount that diminishes for each successive support encountered, natural rock decreases pressure by a fixed amount.
At the same time it would suck if there couldn't be those awesome natural caverns that are currently in the cave systems, so I don't know how to solve that one.
The need to make tunnels would probably increase the time needed to move around in the mines, so mine carts and possibly rails would probably alleviate that somewhat.