Yea, I’m thinking. We could technically specify a country code. So if you put US it will only generate domestic flights. But for this to work this airline cannot be signed if your player airport is outside of US. However, as you mentioned Iceland as an example, you want Icelandair to also create international routes right? So it cannot be domestic only. If you place an airport in norway, you would also want to sign Icelandair. As you can see it gets complex fast. For this to work I think we need to do two things:
Have strict bool for domestic flight only. This airline can ONLY be signed if player airport is within its specified country/countries and will never fly outside of the country.
Implement country codes as a home base which would then make sure the airline only offers domestic flights if the player airport and airline country match. The airline can be signed from other countries but not offer domestic flights.
This required the player to setup its airport in the correct country if the player only uses modded airlines, otherwise there might be an issue getting flights.