I don't have the skill to mod anything, but there are a couple things I feel are missing from the game. I am not sure how possible they are.

1 - Customs/Immigration zone. There is a security zone to get into the airport, but not out. It would add another element to have an immigration zone.

2 - Layovers! Not all PAX should leave the airport. Some should stick around for their next flight.
That leads me to...

3 - Airport Hotel. For the PAX who have an overnight stay, and Airline hotel would be a cool addition to add it.