The most fuel-efficient aircraft possible would be an electric one (for the sake of argument lets assume batteries of high enough Wh / kg would exist) flying at much higher altitudes (20km+) than the current fuel and oxygen fired engines.
That definitely involves both engine and aircraft design.
Even for jet engines: an engine from the 50's is not going to cut it in terms of fuel efficiency on ANY airframe compared to current flying machines. Even if you had unlimited budget and time to design the perfect airframe for those engines.
> (for the sake of argument lets assume batteries of high enough Wh / kg would exist)
Highly unlikely, it's not even close right now, and nothing in recent progress indicates that soon we might have batteries that are denser and lighter by a factor of 10 any time soon.
At this point the 737 Max 8 and A320 both are sitting around an 18:1 L/D. You can't really go a whole lot higher than that without affecting controllability. Things like winglets give you incremental improvements on the order of single digit percentages.
But weight savings... that's huge. For every pound you shave off, all other things equal, means the engine needs to make less thrust and the aircraft itself needs to carry less fuel.
