I have not seen any logic regarding colonies/ethnic or "close" countries (in terms of language for example).
Let me take an example.
The island of Reunion, close by Madagascar (Africa) is a french departement. Its main airport is Saint Denis (RUN). In real life, it has many flights per day to Paris, and also several to major french cities (Lyon, Marseille, Toulouse). The demand is very important, and a company (Air Austral) even bought 2 A380 for this route.
If I look at AirWaySim, there are 280 passengers estimated for CDG-RUN, against... 330 for LHR-RUN!
And 20 passengers for RUN-LYS, while 777 or 330 are frequently connecting the 2nd french city and this french island
This is unrealistic, and makes me think that this is not at all implemented.
This is the easiest example, as it is an actual french departement (like Hawaii is a US State).
But there are more complex situations. Old colonies, but also countries sharing the same language. Demand is much higher between Madrid/Barcelona and South America than from CDG or FRA.
US cities are far better connected to UK cities than the rest of Europe.
Montreal is very well connected to French cities. Same for many countries in West and North Africa (Senegal, Ivory Coast, Algeria, Tunisia).
All (mostly) because they share the same language (which reflects very often an old "colony").
The 3rd thing I'm thinking about is "ethnic" demand. Countries where immigration has been very important, bringing back frequently many people in their native country. It is very often old colonies as well, but not always. But the definition starts to be tuff there!
Wouldn't it be possible to add official language to a city, and give some kind of "boost" if cities share the same language?
That's the easiest solution I see, and that would cover most of scenarios from my perspective.
Going through history is a bit more complex, and I dont even talk about having real data for all the routes in the world :-)
Thanks for reading this