I think that simple planes could use a few things, such as....
rope or cable, so you can carry payloads or make radio and rigging wires plus a lot more.
A timing or sequencing option for the rotators, so you can create proper sequenced gear doors. (Impossible to make gear doors right now on the same key command, because the landing gear is always faster and there is no way to make them not hit without some sort of timing.
And a more weapons. Come on! How long has it been since we have had new weapons to play with????
And last but not least, water engines, with boat props to attach
To the engines like the car engines and wheels.
these were a few of my thoughts, i hope you agree with some of them.