My suggestions that I think would improve the game;
1) Perhaps an upgradable flashlight, that adds a little radius to the area that's illuminated when you walk.
2) A drop option, in case you do like I did so many times, and accidentally mined some iron or coal on your way down for a solid diamond run.
3) A rush option, that would make your character drill in a straight line quickly, for a pretty hefty detriment to your battery.
4) Enemies? So far the only possible way to take damage is by over heating. Maybe just moles, rats, little things to pester you, and as you get deeper the threat becomes greater and greater. Perhaps you could implement a weapon system that allows us to switch between the drill, and a weapon of some form.
5) Day/Night cycle. Purely Aesthetic, no biggie.
6) Waypoints would be EXTREMELY useful for if you found a good chunk of a valuable ore, and you fill up before you can mine it all.
Just little things that people might appreciate.
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