Fuel Burn Increase with age
This is a feature that often gets over looked, but is an essential game mechanic that could change AE drastically. As an aircraft gets older they become less efficient than when they first rolled off the assembly line. for example a brand new A320 according to AE burns 1.78 gal/mi however a 10 year old A320 will no longer burn 1.78 gal/mi it will burn fuel at higher rate, for example 2.0 gal/min. This can have an effect on your profit margins etc, It's also an important reason why low cost aircraft have younger more efficient fleet and go on to sell the aircraft at younger ages.
How would this change AE?
It would change the way people view the used market and their fleet. Fleet renewal is an important part of any airline. Understanding the right time to replace aircraft and the right time to buy used aircraft would also change. If you're a low cost carrier buying new aircraft will be much more important to you than a full legacy carrier. This will also possibly increase the amount of aircraft on the used market for new airlines.