Hydrogen, if a cheap electrical conversion can be found. CNG would be good, as it appears we have alot of it. Biodiesel in a supporting role, but not exclusively as we would have to give up eating to have biodiesel, only.
Ultimately, battery power that can go 1000 miles on a charge on a 4000 pound vehicle at speeds up to 80mph. That should be the next "Manhatten Project", or Kennedy's "putting a man on the moon in this decade" type of effort. I feel it can be done.