I would LOVE to see the addition of some more utility type aircraft that your airport is forced to deal with. This could include for example:
CL-415 Waterbombers refilling to fight off map wildfire
C-295 Military Transport making a stop to refuel/deliver cargo
I think it would be fun if you occasionally had to accommodate something unexpected like that. I really could see the water bomber portion being a fun side event in the game
I think this could go beyond General Aviation though for at least the wildfire one. As a special event, imagine if you had to accommodate suddenly a massive increase of priority airtraffic with time sensitive refueling/refilling requirements. It would give you a chance to have something out of the norm to keep you engaged. It also would be a challenge no matter the size of your airport, as a small airport with low traffic you would see a massive increase but on a large airport with heavy traffic you would effectively lose a runway during the event.
I think that could be incredibly fun, and a way to keep the game from getting too stale.