I like the idea of retractable wheels for the jet. Perhaps ‘CTRL’ could be used to toggle them. Even though it already exists as a mod like TSB mentioned, I think it would be cool to see in vanilla.
Additionally, I previously suggested introducing a new type of fuel for planes and helicopters, along with some ideas for adding more realistic physics. While these suggestions were originally made for Unturned 4, I think it might be possible to implement these features in 3.x as well.