One of the things about ACEO that is not up to par with the rest of the games is the destinations for airlines. I think a good way to fix this is to assign other airports values, and your airport can only provide service to other airports of lesser or equal value.
For example, lets say I have an airport in Dallas, Texas in the United States.
At level 1, I would only be able to have services to other small (level 1) airports such as Amarillo, Lubbock, Corpus Christi, etc.
At level 2, I would be able to have services to medium (level 2) airports such as Austin, San Antonio, Tulsa, etc.
At level 3, I would be able to have services to large (level 3) airports such as New Orleans, Kansas City, St. Louis, etc
At level 4, I would be able to have services to major (level 4) airports such as Los Angeles, New York, Miami, etc.
(Probably for North America only) At level 5, I would be able to have services to international destinations such as Mexico City, Toronto, Caracas, etc.
Airport levels are already in the game via the star system used by airlines and can easily be tweaked with the eventual addition of services such as catering, cleaning, and deicing.
Of course, this system would have to be fixed if a hub system is implanted and a range limit needs to be present for the early levels to avoid 1000+ mile flights between small cities. But the system provides a fix to something I often see people, myself included, complain about.