[MichaelZuo]

Modern high bypass jet engines are already pretty close to maximum possible thermodynamic efficiency, given realistic constraints such as weight, wing clearance, noise, and so on.

Even if all constraints were relaxed, the absolute limit might be another halving of fuel consumption. (with trillions of R&D)

And beyond that, there literally can never be jet engines significantly more efficient, until the end of the universe.

[nradov]

There is still the potential for significant efficiency improvements with open rotor ultra-high bypass propfan designs.

https://www.geaerospace.com/news/articles/shape-things-come-...

And for short flights, battery electric or hybrid power could boost efficiency even further. Although that will depend on developing batteries with better safety and power density characteristics.

[MichaelZuo]

The further halving already includes literally everything under the sun, such as open rotor ultra-high bypass propfan designs.