I want to try to list steps that car makers could/should take to improve the fuel efficiency. The most obvious would be to mold the plastic on the front of the vehicle to be smooth and round, with grill openings that are sized (and placed) only as required to cool the engine. Similar to the original VW Passat (though it could be "rounder" in the horizontal direction):
...or the original Ford Taurus (ditto with the squared off front shape leaving room for improvement):
Sealing all the seams and joints (with gaskets or backing flanges) in the high pressure areas of the vehicle.
Smooth & aerodynamic underside
Tire pressure monitor.
Average & "instant" MPG displays, that are on all the time. (The xD uses the same display for the odometer, the two trip odometers, average MPG, instant MPG -- way too many things on one display! You have to toggle through them with a button that could not be a bigger pain to press...)
Fully ducted cooling system, with the intake down low on the front, and the exhaust in a low pressure zone; minimizing the size of the radiator and the reducing the drag as much as possible.
Throttle control that maintains even throttle in a user-set speed range, with minimal throttle increases to maintain the speed in that range.
Higher gearing with 6-7 speeds (smaller engines need more gears to work efficiently); or CVT transmissions.
Video cameras and screens in place of side mirrors.
Efficient and effective fresh air flow through the vehicle, with intakes on a high pressure area, and exhaust vent(s) out the back of the vehicle into the low pressure wake zone, to reduce drag as much as possible.
Automatic engine shut off and start up; at least with CVT (or automatic) transmissions -- and possibly with standard shifts, based on having both the clutch and the brake applied?
Lean burn in low vacuum conditions.
Store hot coolant in a vacuum insulated tank, to speed up warm-up time; a la what the Prius does.
Coat all the window glass to exclude as much heat as possible.
Make A/C on the defrost setting optional.
Tighten up wheel openings, and always use aerodynamically designed wheels/covers, with rear wheel skirts (at least optional).
Make roof racks removable. Years ago, I saw a "papoose" add-on storage system that locked onto the back of the car, with a single caster wheel to support the weight -- it tucked completely into the air flow behind the car; and it would be a great way to add storage space when needed; that did not affect how you drove very much.
For new 4-cylinder engine designs, the crankshaft could be split with a hydraulic coupling that can automatically disengage two cylinders completely; saving all the pumping and friction losses, for situations when 2 cylinders are enough to provide the required torque to move the vehicle.
Or, make every vehicle with a plug-in electric w/ serial hybrid ICE drive train.