It's not an issue of function, it's an issue of marketing and sales. Companies have found a way to sell less function/$, so they always have a "amazing technical improvement" in their back pocket. If the buyers held back until they had an 1100lb 4 seater with a cd of 1.8 and FE +130/gal, we'd have it. As it is, they are going to give as little as possible and make it sound as big as possible. I'm not anti-corporation here, just a realistic part of business. They sell what sells.
Really? The R&D we need for FE was done almost 100 years ago, but they're not going to provide a "cure" before the disease becomes worth $$$$ to the masses.
So maybe I'm a cynic?
"If you want to save gas I suggest you permanently remove the drivers seat and steering wheel. That seems to help." -Oscar Halverson
Look at all the length required for that aerodynamic rear. It will cost a LOT of weight, which will bring down FE, and it will cost a lot of money too. Looking at it, it looks like you don't get much trunk space for all that length either.
Anyway, considering the success of the PT Cruiser, I bet if they built it it would sell a lot. It's butt ugly to me and I'd hate to see them everywhere (and I don't usually care much about how stuff looks!). Still, a car company might be able to make it very efficient, and if they sold, it would benefit them by offsetting their SUVs for CAFE requirements. Perhaps you should suggest it to various manufacturers.
I'd guess that either Chrysler, with their success selling that era of retro styling in the PT Cruiser and the Prowler, or Toyota's Scion division could pull it off.