Just an Idea for passenger # achievement.
The idea that is you pass a Passenger # Milestone after unlocking the next airport, that you get to build a Hotel from one of the generic blocks at the airport.
For example, I am close to passing the 1 Million passengers mark at Innsbruck. The Hotel would be kind of a Trophy Building. And maybe at later marks you could add a Star 
to the hotel.
At other Airports the passenger marks would be higher but it’s just a suggestion.