You sure can change the stimulus. Three options immediately come to mind. First, you can increase the power with which you explode out of the bottom, that is push at the ground harder and faster. Second, you can pause at the bottom for a variable length of time, adding an isometric element and increasing the difficulty of the concentric movement. Third, you can do one legged squats to greatly increase the weight per leg.

Have your heavy barbell back squats made your legs and core strong enough to do explosive pistol pause squats?