As of Experimental 35, we have international/domestic flights, real airlines in the workshop, regional airlines and lounges. A lot of these ideas probably would only work with a lot more airlines in game or with many reality-based airlines downloaded from the workshop.
Add a home countries/cities/airports property to airlines. Comparing it to the country the airport is located would classify it as a domestic or foreign airline. Domestic airlines would fly to other airports in the country, as well as international destinations (if the airline has the appropriate aircraft), while foreign airlines would only have flights to airports located in their home country (if they have the appropriate airplanes).
It seems like ACEO started implementing this by adding regional affiliates as new airlines. Maybe allow airlines to be assigned as regional affiliate of a parent airline. This would allow the regional affiliate to be scheduled under gates assigned to the parent airline, while restricting the regional airline’s flights to only domestic flights. Alternatively, this could be accomplished by having a regional airline flag for airlines, which would restrict it to domestic/short distance flights, and allow assigning multiple airlines to a gate.
Similarly, lounges could be assigned to multiple airlines, simulating allowing passengers from airlines, their regional affiliate and their airline alliance affiliates to use the lounge.
Allow setting airport as a focus or hub airport for a particular airline. Designating focus city status would increase more domestic flights to be generated from the airline. Hub cities would receive more domestic and international flights.
Why it should be implemented:
For those of us with airlines off the workshop based on real airlines, this would make airport operations/scheduling appear more realistic, domestic hubs with dedicated gates (or even terminals) for just one airline and international flights to correct cities based on the airline.