Induction loop would be the cool way of doing it, and in keeping with the car itself. But it could still have a micro USB socket as an alternative (or sole) charging method.
We could charge it anywhere then, including sitting at our PC.
Most importantly, either would allow the car to charge it, and be driven.
There is one issue though. If the battery dies away from a USB point, how do you unlock the car to get to the charging point?
Perhaps a battery warning indicator on the main key. It could also drop down to a failsafe mode that conserves battery power when low, leaving only the unlock car feature capable of running.