So build us a better engine, and someone will build us a better hybrid to use it. I think hybrid technology is the future, no matter how much you improve internal combustion technology. In addition, hybrid technology will continue to improve as battery technology improves. I expect significant improvements here as well. Finally, let me note that the target of the Prius is not high mileage, but rather extremely low emissions.
High mileage is a pleasant side effect.
At times, the Prius sacrafices mileage for low emissions, especially during the first 5 minutes or so of driving. Cedarville to be exact. It has been in my family since the 50's. If you know any contacts for geothermal power, let me know. I think greenhouses would be the way to go up here.