Add the option of thrust reversers to all jet engines and make sure that they only deploy after touchdown (my idea is to use bucket reversers on the j15 and j50 and cowling reversers on the j90, bfe150 and bfe300). Also make the brakes and airbrakes deactivate after stopping to allow for powerback using reverse thrust
Jarno de Wit commented
If thrust reversers are added for those engines, I'd also like if the propellor engines (Blade TX000) also had an option to "reverse thrust" as an action-groupable setting where the propellors will have their pitch inversed when a certain action group is activated. This would be very useful when the VTOL control (and the TRIM) is already used by some other feature on the aircraft, and thus no sliders are available (on mobile).