just had an idea that when buying another terminal that its takes so many game days before you can use it as i think in my opinion that its to easy and to quick to expand your airport.
But that will be no fair to new players (compared to those already have all developed). As a principle its not bad idea but put yourself in shoes of “beginner” and that perspective makes it quite upseting (I guess).
keep all current airports the same so new players can get the hang of game macanics and then just bring in the delayed at furture airports.
or another way is give players the option when entering a new airport.