Whenever I get practically ANY game, my brain seems to just churn out ideas for mods or future features like a factory on steroids. So... here's another mod suggestion.

Persistent objects/aircraft.
This could be an option (exit, exit and keep aircraft) which means that whenever you next fly a plane in sandbox mode, the previous aircraft you flew (and any other saved persistent objects) will still be in the positions you left them, at the velocities and states you left them, basically flying/moving/staying sit with no AI. Plus, maybe an option to switch aircraft. So, you can make one aircraft, fly it around a bit, park it somewhere or put it on a certain course and then fly a new aircraft, with the old one still there.

One example could be if you set an aircraft on a collision course with the USS Tiny, packed with a load of boom 50s, and have to use another aircraft to shoot it down before impact.

This may be one of those things that is not possible even with mods, maybe due to how things load and unload between flights, but if it is then it may be very interesting to see. Possibly an option to have an AI take over aircraft you've switched away from (with you probably telling it where you want it to go or what you want it to do). The switching may be impossible due to how the game is coded but, if it can be done, it might be a good concept to have.