It truly does seem like the craziest ideas sometimes turn out to be the most popular, but I guess that's because the people behind them have a creative vision of the future, aka true entrepreneurs.
One of my concerns for FlightCar is the amount of competition they face in other cities. Expansion could be tough but again, their prioritization of customer service is a great initial platform whereupon users will be naturally attracted to using their service.
One idea I had was to create a rewards system such that people gain 'points' by putting their car up for rent. And once a number of points is accumulated, prizes are offered or something of the sort. Could be a neat marketing tool as well as an edge on similar services.