The idea is perhaps not the first that comes mind. But what if you could 3D print fuel. Control its properties and composition completely or at least more than now. I got this idea from thinking about different forms of coal. If you could enhance them before combustion. So to get a higher highest temperature. For higher efficiency that is. I also took inspiration from the idea of creating 3D printed metal with very optimized properties.
So the idea is to make optimize our fuels like coal by 3D printing.