Only metal foam crumple zones can provide the biggest mpg advantage!
 

IMO the biggest factor affecting mileage is vehicle weight.
Unless you use exotic materials a lighter large-sized vehicle will tend to be fragile.

Thus we need rollcage style rigidity around the cabin. No problem here so far as race homologated rollcages only weight a few hundred pounds, yet they withstand spectacular accidents.

The problem comes in crumple zones.
Even if optimally shaped sheet metal is a poor material for crumple zones: not elastic and heavy.

This is where foam metal will come to play.
They offer exceptional shock absorbance for its light weight and small volume.
For example you can google titanium and aluminum foam crumple zone reasearch.

Thus IMO the mpg futures is in foam metal crumple zones.

Cars have been doing this for 20 years plus. Plastic and foam have been front and back of many cars, including hondas, since the early 90's.

The weight of the crumple zones is not very much.

Foam crumple zones are designed for low speeds.
They offer little protection at 55mph or 110 mph (you are traveling at 55mph and you crash head on with an incoming 55mph vehicle).

I disagree with weight being the most significant mpg factor. Regardless of weight, my mpgs don't fluctuate much. In stop and go driving weight is the supreme factor, but at any sustained speed, aero is king.

That said, I still would like to see weight reduced because vehicles are more peppy and fun to drive the lighter they get.

Drivetrain efficiency is most important. Second is aerodynamic drag. Third most important is weight, and last is all other rolling drag.

weight is THE easiest changed factor in the first, third, and fourth item in your list.

also interesting is you don't have accelerating the car as significant. Must be a pretty nice commute where you never have to accelerate the car at all.

You know the energy needed to accelerate isn't lost, right?

Right -- weight *helps* with coasting, and if you have an EV or hybrid, you can also "get back" some of the invested energy in acceleration. Also, acceleration is much worse for an ICE powered vehicle (they lose about 75% of the energy vs about 10-15%), so for a "city" vehicle you should have an EV and light weight, with aero moving down to third.

Back on topic: a larger crumple zone with lower weight and good aero all are desirable. But if you throw it all away with a low efficiency drivetrain, then lower weight is more important.

As traffic keeps increasing most driving will become stop 'n go.
And I agree with lightweight.