How about a tri-hybrid. Diesel electric base operating system with battery storage for converting breaking/idling energy back to usable energy. Then slap a small steam generator on top of the diesel engine to convert waste heat into electricity. You could even throw a couple solar panels on top for Greeny wetdreams. It would be a horribly bulky system, but you would be much more fuel efficient.

I wonder how much of fuel efficiency is related to aerodynamics. These big square faced vehicles have to be wasting a ton of energy overcoming air resistance. It always seemed to me the best mode of transportation would be something like the Messerschmitt KR200, only sleaker, and give it a steel cage so a driver has racecar like survivability. Watching hundreds of 2 ton trucks driving down the interstate with only one person inside just seems so wasteful to me and it's all predicated on the belief that SUVs are safer in accidents.