Fusion power. New scientist last week put a 40 year date in the future for when fusion should be a reality (after all, we do know fusion is not just theory, its one of those technical/material limited problems).
With the promise of literally unlimited supplies of energy, with the only by-product being very limited low level radioactive waste, and the chief fuel lithium (enough in 1 rechargable battery to supply an americans energy demands for their whole life), the only problem currently is nations are unwilling to stump up the cash for the research. But as soon as national economies and strategy are threatened by fuel shortages, I am sure this source of energy will recieve the cash it requires. The curently envisaged research fund will require 10 Bn per nation involved in the project (6 currently) to get the prototype reactor going.
Once we have the energy, either efficient fuel cells (hydrogen fuel, storage however is an issue, the density of hydrogen being a hundredth of avtur) or synthetic ethanol (little modification required for curent engines) are viable future aviation fuels. Not that I am fussed about the fuel issues, its aircraft automation I worry about!