How about a pressure sensitive horn... The harder you push the louder it gets?
It's a good idea and it could work, but for safety features like a horn I think it should be as simple as possible when it comes to operation.
I seem to remember the Lightning Car Company (Welcome to Lightning Car Company - The UK's Premier Electric Sports Car) in the first version of their website "offering" external speakers to make ICE noises on their supposed EV as well.
I still think PV on the roof does make a bit of sense. But only to drive the AC within the car/battery during hot summers days when that problem is important. That way you can probably sit parked somewhere in the sun and keep the temperature at decent level inside the car and not have to "turn on the engine" as you would with a dinocar (well use the main battery in this case but you get my point).
PVs on the roof are a great feature. I know they have something in the area of 30% efficiency and perhaps someone could shed more light on this but in an EV application would it be able to collect enough energy to regulate the temperature of the battery pack on those hot sunny days (when parked)?