I would like to suggest the addition of cargo varients of aircraft into the game as well as cargo airlines. We could also add a option of if you want a: cargo only airline, passenger only airline or a mix. Of course to stop everyone from taking a mixed airline we could have bounuses for airlines that are only cargo/passenger varients.
The cargo aircraft would most likely have to have their own gate/hangar system and prices (for example; price per ton, for live animals, delicate cargo and so on).
I think this could add another level or realism to the game and make it more enjoyable