Here are some things that I would like to see added:
- ability to fill in tunnels
- ability to place and build structures in tunnels
- reinforced tunnels last a long time, and tunnels can be maintained (without too much work) to last forever
- ability to lower and flatten stone blocks, so that mountains can be terraformed
This would add so much, and I personally know several people who would be much more likely to buy the game if there were a better tunneling experience.