The beauty, from an engineering point of view, of the EREV (range extended electric vehicle) architecture, is that in theory the range extender technology could be adjusted to be the most efficient or cost effective, without changing the essential function of the vehicle. For example, if a new engine maker such as Cyclone Power or EcoMotors, or perhaps some future Sterling engine developer package an emissions compliant powerplant which could generate enough electricity to sustain an EREV, we could see vehicles which burn different fuels than gasoline competing against the old gasoline ICE.
The problem with EREVs, of course, is that they are very complex, requiring both electric and combustion systems, with their separate controls and sensor networks. As Fisker and GM are finding out, this is not easy to pull off cleanly.