The way I see ACC being used is how quickly the player goes from slower speed to higher speed - no matter where higher or lower is. So if I'm going from 30% to 60%, or 20% to 100% - ACC = the rate the player actually gains velocity. So this brings in actual physics. We have his max speed and his rate of acceleration.
In real life, when a player starts running, be it the 40 or an out route or a juke, the player is going from a slower speed to a higher one. How quickly he does that is his acceleration ability.
So on the snap, let's say I'm doing a run play, the QB hands the ball off. The HB is in motion and now the circle is under him, so it's on me.
If I don't touch the stick (and assuming not in a coach mode), he'll come to an eventual stop/very slow speed (which might be useful, too...perhaps to get those chop steps/stutter steps in the mix - letting go of the stick would be the easiest way to do that from a control standpoint). If I do move the stick, he'll go towards a speed based on how hard my stick goes.
If my stick is about 50%, he'll go at not quite his full speed, just like in, say, Warriors Orochi 3 where I can make my character walk by pushing up half way, or go full speed by pushing far enough up.
So the PS3 obviously can tell a difference between the levels of the stick. I notice this as well when I use my stick with my PC emulator. It registers between -1 to 1 on both the X and Y axis.
So I'm envisioning using the L stick for this. Basically the L stick sets the "goal speed" of the player. Pushing on the accelerator is you moving the stick forward. Steering left and right would be moving the L stick left and right. Slamming the break is pulling back.
With this, we could do ALL moves of a halfback. A juke would be a quick hard left then full forward-right, having my player wanting to explode back to full speed after stabbing his foot into the ground. A stop-and-go move would be pulling back, then pushing forward again. And so on.
ACC would play into how quickly the player accelerates up to the speed. So a guy with 50 ACC trying a juke would not explode up to his top speed as quickly. He'll get there eventually if given enough space and time, but vs a 99 ACC, the control would feel more "clunky". That might sound like gimping the user, but it causes the user to change running style just like you don't drive a SUV like a Formula 1 car.
AGI could come into play here, too. That's like the "handling" of the car. A SUV has poor agility. That would be like a FB trying to cut like Barry Sanders - probably not happening. At more subtle differences, it creates separation not by "delaying" reaction, but because he can't "corner" as smoothly. He loses time and distance while the nimble athlete whips around almost without breaking stride. Now we have Jerry Rice vs a faster, but less smooth-footed DB. And then it brings in more user skill WITHOUT overriding ability. User skill becomes about melding with the player you're using.
STA of course modifies energy of the player and is like the gas in the tank. As it dwindles, SPD, AGI, ACC all suffer. Top speed wanes, the cuts get a little sloppier, the explosiveness wanes.
That's a lot of how I envision just the player's abilities doing the work without needing a "speed burst" button.