More Mining Equipment
As someone that's in the mining industry, it would be cool to have more equipment like dozers for stockpiling and landscaping along with drills and blasting for making loose material that loads faster than when the shovels are digging into the hard face.
Also maybe adding a lube truck to supply the larger mining excavators with oils and grease along with fuel. Graders would be nice touch too to improve roads and speed up travel perhaps.
GOING UNDERGROUNG MINING WOULD BE THE COOLEST. Ability to add portals and shafts with an underground map and underground mining equipment. Would be better for late game if you had to build a lot on top of minerals in the start.