Really efficient solar cells to help store energy to a better developed battery system.
Making bio-diesel out of Hemp. This way it doesn't affect food costs (well, there is hemp bread in publix, and other hemp products).
Using the oil from all the fast food restaurants for bio-diesel as well.
Better recapture of expended energy. UPS is using experimental trucks that store compressed air when they brake and use that to boost take off ...
so continuously storing and releasing energy.
Using electric vehicles that have motors in the wheels, instead on a driveline. This reduces driveline losses accumulated by the transmission,
differential, driveshaft, friction, flywheels, etc. Having a small generator keeps the batteries charged up, but, also regenerative braking.
Perfecting the use of Hydrogen or other gasses as a fuel source, that can burn cleanly ... making the production of the resources more efficient and
bicycles ... shoes ... skates ... just a thought
Absorbing kinetic energy that could be created without effecting the car much ... hydraulic system with a water wheel type device, a thick liquid
transfers between cases pushing a wheel to create energy ... gravity/centrifugal force powered ... through turns at speed, uphill/downhill slopes,
acceleration and braking.
some are a little out there, but, sometimes thinking outside the normal realm can help others think further and create something useful.
I don't think any one solution is best, but, combining multiple ideas to make one really great package, though it does help if there is independence
and redundancy in the system, so one part can fail but overall it is still usable until it gets fixed.