How about filling the wings with hydrogen, and the plane gets heavier as you burn it? Then you could land heavier than at takeoff.
Swissair reportedly shut down one engine at cruise in their DC-9s. It was not just flight idle, but complete shutdown. Wasn't the center engine shut down at cruise on the 727 with re-engined #1 & #3?
If you want greater efficiency, charge SLF by the pound, like normal cargo.
GB