Extendable Car Bumpers (a free* idea)
Many people are poor at getting in & out of parallel parking spots (including me!). Further adding the problem is the paranoia that when you return to your car, you have been "boxed in" by cars in front of and behind you.
The stop anyone boxing you in, cars could have bumpers that have a lockable, extendable attachment, something similar to steering wheel locks:
After parking, just extend it the length you require to get out of the spot, and when you return, unextend it!