I'm surprised nobody made a mod like this yet unless its not possible at the moment but I think it would be an excellent idea to create planes that import and export cargo from cities and build a terminal like the train or cargo ship terminal to go along with the cargo plane. I messaged BloodyPenguin the idea, he was thinking about it for a long time but unfortunately he doesn't have the time to make it. Just an idea but would be cool if someone made it.