The long-term future is fusion... but it requires money to be tossed at it before there are breakthroughs for sustained yield on a level that can be used for 24/7 energy protection.
Mid-term future is nuclear fission, namely thorium reactors. However, the word "nuclear" causes untold panic that people would rather tell their sons stories about those levers on the wall that show "on" and "off" than move to an energy source that can handle future growth.
Near-term, solar and wind are nice... but with the politics as they are, we will still be using gas, oil, and coal for a while still.
As for engine design, since we have common rail fuel injection and very precise metering, it is only a matter of time before someone designs a diesel engine that can burn not just oil, but gasoline, natural gas, and propane.
We also have had turbine engines going on a century now. Turbine engines can run on gasoline, methanol, ethanol, natural gas, propane, diesel fuel, or whatever you can throw at it. This probably is the future of engine design because it can handle virtually anything thrown at it, and who knows what fuel will be cheap and useful in the future. We might end up with a government push back to ethanol, or maybe even wood gas.