I started a project about 9 years ago that was a 3rd brake light, that lit up more the harder you were braking. It used an accelerometer so it could be aftermarket but I'm sure there are better ways to do it if it's built into the car.
I know some German cars have the lights getting brighter during hard braking, but I think it's more intuitive to have more lights coming on, so for example you could have an LED strip across the top of the screen that if you decelerate AT ALL (using gears for example) you get the 2 middle LEDs lit up, then a bit of braking gets 4 LEDs. If you slam the anchors on full you get the whole lot lit up (and hazards flashing for good measure).