Range extender option
No don't make it like Volt.
Have an option of towing a trailer with a generator on it.
The generator would automaticly start when the battery reaches 2/3 discharged level. When the generator runs out of gas you simply drive to the closest gas station for a refill.
The advantage of this you could buy the generator or rent it. Just contact your local Ford dealer to reserve the extender generator before your trip. This way your not hauling around an engine when you don't need one.
Ford would have to add a power port for the generator to plug into. The blue tooth could be used to remotely control the generator.
To configure the car for range extention could be an option, or dealer installed.