I think every country should have a "flag carrier" that is the airline currently serving a particular country "the best". There could be bonuses attached for alliance score and achievements included also. IRL the term is used in different contexts, but is often the state run airline, but we'd be using it to declare who serves the country the best.
I'm not sure how all the numbers are tracked on the backend, but there would need to be several benchmarks used as the flag carrier shouldn't simply be the "biggest" carrier, otherwise RyanAir would be the "flag carrier" of Ireland instead of Aer Lingus. It would have to be a balanced scorecard approach where connectivity and domestic destinations are included. The denominators would be the best in the country for that one metric, so it would look like this:
score = (airline pax)/(country airline with highest pax)
* (airline route pairs/direct flights)/(country best)
* (airline domestic route pairs/direct flights)/(country best)
* (airline CI)/(country best)
* (number of airports served)/(country best)
* (number of staff)/(country best) [this would be in place of fleet size since large aircraft require more staff than small]
This means a small domestic airline could earn a higher score than a large international airline with sparse domestic coverage. Flag Carriers could also have their country flag displayed by their airline name. There could be other benefits for being the flag carrier, such as a CI boost or when city-based demand is implemented there could be some other sweeteners. When terminals are implemented there could be other restrictions or benefits given to flag carriers. For long term players, there could be an achievement where a player has to finish a game world as the flag carrier of the top 10 countries in the world (would take 10 game worlds). If they accomplish this, then they get to play AWS free for life (or some other benefit). This would also keep players in the game until the bitter end versus pulling the plug early.
Top 10 Countries to get Flag Carrier status (can get one or the other, in no particular order):
United States or United Kingdom (largest airline markets in the world)
China or Japan (major Asian countries)
France or Germany (large European countries)
Italy or Spain (also large European countries)
Singapore or Hong Kong (city-states)
Korea or Malaysia (large Asian markets)
Egypt or Algeria or South Africa (major African countries)
Canada or Australia or Russia (vast, sparsely populated countries)
Brazil or Argentina (South America)
Wildcard: Turkey, Thailand, UAE, India, Philippines, Taiwan, New Zealand, Mexico, others?