The game obviously needs roads, and I thought a neat implementation of them would be asphalt. Since we get the tractor and depot when we unlock coal.. I thought it would be worth adding on more to that tier. Bitumen is found with coal, so the idea that we could make an asphalt plant that combined limestone and bitumen (or coal) to produce tar isnt far fetched. You could also make a truck to spread the asphalt along a path. The asphalt would destroy smaller rocks, and (if possible) adhere to the lay of the land. I think it would be cool to add another layer to the vehicles, with some construction vehicles like this.
Great game, and thanks for the effort!