Make a small, light sleek fast 4 door economy car. Very Low displacement flat/straight 6 diesel rear/mid engine, 8-10 speed manual with assisted auto shift or replace with a generator and electrically driven wheels.

Why small and light? Less weight, less cost to move, less rolling resistance

Why 4 door? Because you know, things like the Subie Impreza are fun, yet practical

Why sleek? Wind drag coefficient as many cars today look the part, but are basically rounded bricks and don't slip through the air. Remember the F/A-4?

Why a very low displacement straight 6 diesel? First off torque. Small, compact enough to be able to build to fit into places. Second small displacement means less rotating mass, less vibration, less fuel consumption, higher rpm limits more easily. If going electric generator, motor needs torque from the get go.

Why an auto-shifted manual? Efficiency. Smoothness, short power band for the motor needed... perfect for High Efficiency...

Why possible electrically driven wheels? Bio-diesel with generator and electric motors... what could possibly be bad from this in an age where petroleum from the ground is effectively going away. Also, given the fact that a possible way to produce Diesel and JP fuel from seawater... for similar to right now current costs of at the pump fuels... no brainer. Also, less batteries, being used for starting and gap power.

There are other things... but this would be the most looked at one.