I wouldn't mind some kind of mining system. Mine ore, send it to the factory. If not enough people keep mining, the costs of weapons explode. You get paid something to, and you can get in fights with other workers. If you find special stones, such as opal, gold, and granite (models already in the game too), you can keep them and sell them for a cash bonus or build something with it or make a sculpture or something. Add random events, like if you stay in for more than two hours you die from coal dust, or having the air randomly explode every once and a while. Then people can host rescue operations...
That would be preety cool. Sounds kind of hard though.