a very interesting topic, i think mod would increase if we didn't have 2 stock classes (21.5/13.5 or 17.5/10.5 depending where you live). Both classes are so close why even have both? This might bring larger numbers to mod, becuase for many good drivers a 17.5 or 21.5 motor is too slow to be challenging.
My opinion on the mod motors is move to rubber tires as the limiting factor, imposing a control tire at big events. Tires are easy to control and monitor, leave the mod motor, speedy etc open. Driving skill also includes having to control a big power motor on a small tire. The fast guys in mod already know when to run a 5.5t vs. a 4.0t motor, the new guys will learn laptimes are not just about raw power. Especially if you only have 3 sets of tires to manage over 10 races.