I think most players who have played this game extensively have felt the frustration of not having an aircraft type that is able to serve good pockets of demand due to airport infrastructure level being too low. I think adding in a button that could allow players to pay to level up airports would be a good feature, as it would open up more gameplay options, and give larger airlines something to spend cash on.
While under-levelled airports don't cause large problems for passengers, as In my opinion serving lvl 1 airports with only smaller jets/props is a very interesting dynamic to the game, in cargo, the infrastructure level is completely gamebreaking. It makes the 757PF mandatory for all cargo focused airlines as serving lvl 2 airports represents nearly half of all cargo demand, and the 757 is the only decent large cargo-plane with global range until the 737-800f/a321f, which is only added in 2017 once most of the games competition is already dead.
I would love to try an airlines based around VL freighters, not be forced into 757f every single game which I want to operate cargo, and allowing players to upgrade airports would be another way to do this. Also, it would make the game more interesting in slot restricted airports because the player could actually do something to increase the slot count instead of wait ten years for the airport to be upgraded. (The current limitation of maximum infra level to game year is really bad and boring IMO).