Not a chance. Auto companies would love to develop more fuel-efficient engines that have just as much power, reliability, and durability as current engines. What you invariably find with these garage-built engines that guys claim can get 70 mpg, is that they don't actually get 70 mpg or they lack the power, reliability, or durability of current auto engines. Gas-electric hybrid engines are for real though, and the prices of these vehicles will be dropping as production increases. That's the wave of the future, at least for the next twenty years.
The same holds true to factory produced engines that don't hold up to the antiquated EPA mileage. The mileage on the Mulroney stickers of new cars are typically off by 20%.