The most practical basic concept for a minimum-impact American car would be an all-electric vehicle that incorporated a fuel-powered generator that kicked in on long cruises when the battery craps out. The Chevy Volt is an early version of this. The common hybrid seen on the roads today isn't a true hybrid, but a gasoline auto with a battery-powered "boost" motor.

If the hybrid's generator is designed in a modular fashion, along with the battery pack, you have a concept that is hugely flexible and upgradable. A generator runs at one basic speed, unlike a standard car engine, so it can be optimized for that, reducing fuel consumption. Generator motors could be designed to burn a wide range of fuels. A gas turbine system will burn almost anything.

As batteries become more efficient, less time will be spent using the generators. Cars used for short haul/commuter applications may end up burning no fuel at all.

I don't see a down side to this.