I have been thinking about how we'd implement this too.
IRL airlines just call airports a hub. It has no requirements and I think it doesn't benefit them.
It's just useful for organization.
However, I don't think that's enough gameplay for our game.
Of course, organization wise, list them alphabetically at the top on scheduler lists. Maybe give them a little icon.
Possible benefits I can think of are these:
-a small reduction of slot prices (5%?)
-If slots are scarce at your hubairport, you have a higher chance of receiving the slot. (maybe until you own 60% of the slots?)
-Passengers are a little more likely to connect through your airport
Possible requirements I can think of:
-5% of your flights pass through the airport (so theoretically you have a maximum of 20 hubs, in practice this would probably be 6-8 hubs)
-for passengerhubs: ticket office and check-in desks?
-for cargohubs: cargo handling center?
Then, I'd also like it if you'd add focus airports.
No requirements, no benefits. Just give them some other logo (or a smaller version of the hub logo) and list them in scheduler lists under hub airports.
Just my 2 cents.