Without doubt the greatest alternative fuel future ever suggested was the one at the end of Back To the Future, in which Doc Brown stoked up the reactors of his time-traveling DeLorean with trash.
Absent that technological development, the car companies are busy flogging other visions of what will follow the internal combustion engine and gasoline as the world's primary energy combo.
Long term, the smart money is currently on fuel cells powered by hydrogen, since that permutation represents about the lowest tailpipe pollution score yet imagined and has technical and financial issues, which can probably be addressed. Inasmuch as gasoline is a finite resource (though one with a confusingly indefinite life), this is good news for our penchant for mobility.
People who want mobility with a little panache are not so happy, however, since no company has yet to show much interest in addressing the issue of fuel cell performance.
I'm repeatedly told that yes, yes of course fuel cells can be tweaked and twisted to deliver more torque and horsepower and different power grades and all of that, but there's been no hard evidence that this is true.
For the average car company this is not a pressing problem, since they are concerned with finding a power source that will keep them in the transportation business and if it helps the environment in the bargain, well that's okay, too.