Building a mine is a good idea. I also am also really liking the idea of monsters coming from the mine. You could have it collapse after a certain about of metal/rocks are mined so you have to build a new one. It would be expensive to build (basically trading wood (for support beams) for metal). After it collapses your miners have to spend time clearing it before you build a new one.
Your PS gave a pretty fun idea. Right now I have a hidden dev menu ~ that allows you to build stuff that you can't normally and for free. It also shows debug information. What if I had a dev tool bench. So you had tools (like normal ones) that did dev like things. Spawn skelly, teleport, kill things, spawn resources, invincibility, resurrect, ect. Haha. I think that would be a neat thing to add just for fun. Obviously not an object you can build normally in the game. Maybe you have to enabled it in a file outside of the game.
Thanks for your post! You are right, I honestly like hearing from anyone who plays and lots of times I add the things people suggest if I feel like fits the 'spirit' of the game :) being a solo indie dev doesn't mean I have to make this game alone.