[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.