I had a suggestion about realistic routes back then:
So basically the idea is that modders can create a list of countries and/or airports that mod airline flies from. When you combine the idea above with current flight generation rules of game; you are supposed to get logical but not 100% realistic routes. Although it is still way better to get an Air China flight from Buenos Aires to your airport in USA.
This also creates a possibility that an international route by a domestic airline can be made. For example United Airlines flight from Guam to your USA airport.