When you look at an airport's main page, it lists a lot of information, including demand calculations, limitations, runway sizes, and even how many flights come out of that particular airport from each company. It notates who your airline has interlining contracts with. What I'd like to suggest is another notation, like the green interlining contract symbol, which notes which airlines have their main hub headquarters at that airport.
The information would be easily available with a lot of mouse clicks, but it seems to be important information to the players, and also to the "airport" that would want to publicize that information.
So, largest number of departure in an airport doesn't mean the airport is the main hub for the airline. It may be the secondary hub of the airline, but it is the largest carrier in the airport. It's hard to define an airport as main hub even though the airline has its HQ there because they can usually build their HQs anywhere in the country of traffic right, but focus in other airports within the country. One more thing is that maybe a country doesn't have a big carrier with many departures, usually nearby countries fly frequently to the airport and it may in turn define as largest service provider or focus hub of the airline.
Well - if you declade the largest number of departure per airline as their (main) hub, there will be a lot of small airlines (especially young ones) which are fulfilling this, but they are not the ones you want to see there. If you let this set by the player manually some airlines will have hundreds of hubs. So I'm concerned that you do not find the information you expect.
I wasn't thinking of a user-defined notation. Here's a little more in-depth of what I was thinking: the airport that holds the airline's most departures is the "main hub" that would be noted on the airport's main page. Whether that number is 100, 1000, or 3500, if that is the largest number that appears on the Office Overview page, then it just has little blue 'X' next to its name, like the way interlining partners are noted on the airport main page.
A user-defined system, as SK says, would be just a lot of airlines choosing every single airport as a hub. I could probably come up with an idea for a hub declaration system. Just off the top of my head, you could allow players with over 100 flights from an airport declare that airport as their hub, but they are only allowed one main hub. Like the maintenance, they have to wait a given amount of time to change their hub. When they reach 1000 flights from a given, they can choose to declare one airport a hub (like above), but they could also note the 1000 flight airport as a "main hub", like a second ranking. This, then, would need to have its own spot on the airport main page, and the original idea wasn't to gum up the works like that.
You could go a lot deeper into it, with declaring airports like "continental hubs", "regional hubs", "focus cities", etc. I could write up a full proposal if it would be worth the time.
I mean, what advantage should it bring to select a particular city as a hub?
(given that you propose being able to change hub every now and then, for example)... e.g. selecting maintenance provider results in different maintenance ratios and different maintenance costs. What should the option of selecting a hub be able to achieve?
Or in other words, should this be a decorative feature, or functional (function-oriented) feature?
Ah. I was thinking it would be more decorative on the AS side, but from the player standpoint, it would help people know what airlines are based at that particular airport, and help keep track of airlines when they go out of business or when they switch hubs.
There could be functionality put into it, but not sure fully what that would be. Perhaps an increase in demand? An increase in flight ORS rating?
Also, just a thought from your post... what if maintenance providers had to be hired per-airport? Maybe some airports don't have all the maintenance providers available... and the costs of maintenance would be determined where the plane was located when a maintenance cycle took place. Would make things a lot more interesting in scheduling and determining who you should have as your provider in each airport. Probably a whole lot of work... but just an idea.