Don't ban aftermarket exhausts completely, just the ones that optimize for loudness or dirtier air.

I'd like to see devices that detect when a car is running too rich or lean (bad cases I can smell right away, so it should be detectable at a range), along with enforcement and seizing vehicles where they deliberately mess with those, especially if there's a switch or function present that can switch between legal and illegal modes to pass emissions tests and then go back to spewing out unburnt fuel or a much higher number of nitrous oxide compounds.