Used Aircraft Leasing
As I was going over my roleplay for my airline, I realised an important feature available to airlines around the world isn't in fact in the game: a used aircraft leasing market.
Indeed, as planes get older they may sometimes be leased instead of sold to other companies. Therefore my suggestion would be to have a used aircraft leasing system, however it would have a couple restrictions in order to be more realistic:
-The used aircrafts to be leased could not be too old as to become redundant, so probably earlier aircrafts before the 1980s should not be available. On the other hand, Airbus 300s/310, MD80s, Fokkers, older Boeing and Airbus such as the 737-300, -400 and -500, 747s, Saab 2000, BAe 146/ Avro RJ, etc. could all be introduced in this market.
-One could lease as many used aircrafts as one wishes instead of 'handpicking' them as you would for the used aircraft market (where you have to order one particular aircraft at a time).
-Newer aircrafts could also be available, whereby instead of a leasing a brand new 737-800 one might select an older model from say 2001. The concept of the used aircraft market would then be to obtain either older aicrafts more quickly (and more of them) by leasing them or newer aircrafts with high mileage at a lower lease price .
-Frenchie