I again thought about some suggestions (hope nobody mentioned them before, if so -sorry)
I thought about the destinations that the arriving planes come from. It would be nice if it would match realistic destinations. Planes in Germany e.g. often fly to Spain, France, U.K. It would be cool that if you have a German airport the first planes that arrive at your airport come from these destinations. If the airport grows more destinations could be made available.
I would also enjoy if one could actively ask airlines to fly to your airport instead of just getting offers from the airlines. I know that smaller airports actually approach airlines sometimes in order to tell them how great their airport is.
Another nice add-on would be if it were possible to also (in addition to the fuel trucks) have fuel pipelines in the ground so that every airplane could be refueled without the truck if one invests more in the airport. I know that a lot of big airports have these systems.
Lastly, I must say that I'm not to fond of the design of the airport busses. In my opinion airport shuttle buses look more like this: https://www.airportbus-muenchen.de/fileadmin/user_upload/kurzer_fussweg_2.jpg In the game they more look like American school buses.