This sounds like a good opportunity for a startup. You wouldn't even need to establish a counter at the airport. As a customer, you just check-in via Blackberry, iPhone, or even text message. The shuttle bus swings by to pick you up and you're off.
Somebody'd better do it, or I will. ;-)
Higher-end hotels have systems that do this kind of thing already, and there's no reason that rental car companies couldn't also.