I could see a number of neat sound options:
Harley Davidson Motorcycle
Ride of the Valkyries
Star Wars Theme
Theme from Jaws (both fun and a great warning!)
In reverse - the standard Beep, Beep, Beep that we all know and hate (but it gets your attention).
If we are going down this road, then the new regulations need to apply to all vehicles, regardless of how it is powered (gas,EV,pedal power).
Personally I think it is just a knee jerk reaction to new technology. People need to be aware of their surroundings and take responsibility for themselves. If you are clueless enough to walk in front of a moving car because it is not loud enough, Darwin will see you to the exit door soon enough for a cause unrelated to EVs.
Perhaps we need some specific standards for this noise...otherwise everyone will play a different tune like in cellphone rings...specifying the dB levels in not sufficient...people need to know it is car coming!
It is blind people who pushed for these minimum sound requirements. I am a blind engineer, and know that this problem has been under discussion for more than 10 years. In Montgomery County, Maryland, pedestrian safety campaigns recommend that pedestrians "make eye contact" with drivers. Good luck getting me to do that. Sound from an "idling" car is just as important as from one in motion. As much as possible, we need to be able to hear vehicles which may collide with us as we cross streets. I think manufacturers are going to be sensible about the sounds they select; a lot of groundwork has already been laid.