Recoils

You are a gun on a parachute, and the only thing that moves you is firing. Every shot kicks you the opposite way, so aiming down is how you climb and clearing the robots in front of you is also how you dodge the ones behind. One button does all of it. Bullets bounce off the edges of the screen, combos pay for chained kills, and every three hundred meters a boss arrives whose armor has to be shot off before anything else counts. Recoils is the smallest idea here and the best executed.