Blue sky thinking?
If you limit me to IC engines: Hydrogen, preferably from hydro, solar, wind or some other "clean" source (nuke would be OK, just have to figure out how to handle the waste). Oh, and for hydrogen storage a low pressure storage vessel filled with some sort of molecular sponge material so you don't have to deal with hydrogen at high pressure.
Ultimately? Super efficient, long life, electrical storage cell (i.e. super battery). Super efficient solar cells incorporated in the hood/roof/trunk surfaces of the car to do the bulk of the charging. Super efficient electric drive w/near 100% regenerative braking. Let's heat it and cool it with a super efficient heatpump (maybe a little, just little, IC motor for when it's cloudy, too cold, etc).
And, no I'm not an environmental freak. It's just that after 30 years of driving, you start to think about all those small quantities of gas that you use to get yourself to work, get the kids to school, go get a loaf of bread... it adds up, then you think about the gazillion other people doing the same thing (and China's now comming on-line) and you say to yourself "How long can this last?"