since more than 60% percent of the energy from the combustion process is lost in the form of heat I think that would be a good place to start. Maybe some sort of exhaust gas powered air pump. You could call it a turbocharger, shit they already have that. Maybe you could use the wasted heat to create steam and turn a small steam engine connected to the engine via a belt on the pulley, of course bmw owns alot of the patents for those devices. How about a six stroke engine like crower (the world famous camshaft maker) created and sold the patent. Maybe a solid state alternator could be used to help relieve parasitic losses from the engine, damn I think toyota is sitting on that patent. Well it seems like all of the really good ideas have been bought by major companies and put on the shelf.