I suggest to give out a golden key for unlocking all arrivals and departures on a destination.
As long as you don’t buy them GPs are hard to earn. This would be a little unfair for those who have less or no GPs.
When you make it both ways the S and M arrivals / departures are cheaper than a GP.
Golden keys, not planes.
Actually, I don’t think that is a good idea. Once you reach a mid point in the game you always have enough money to buy a new spot to send flights to. Just use the golden keys judiciously and open only those airports first where you can send your aircrafts for a lot of money. For example you would like to unlock ADD - Addis Ababa in IAD airport first as when you have your own M fleet, that is the longest destination M flights can go beside player airports. So by the time you get enough M aircraft, your ADD will be fully unlocked and upgraded. I always do this. I only use keys to unlock initial S and M type airports which I require, rest all I purchase from money as when I require.
When you unlock the last arrival or departure on destination, you still earn silver keys. So, I think, it would be nice to spend them to buy another tier on same destination and, when all arrivals and departures unlocked in all tiers, you recieve the golden key.
I suggest this option to get rid of a situation where you have keys, but there is no way to spend them.
Oops, sorry. ¯_(ツ)_/¯
Unlocking destinations is used with golden keys, it would be pointless
