A possible idea for more playing strategies and long term appeal:
Introduce special traits. Each airline could choose 1 special trait at start-up and develop it (improve it) over time.
For example:
-Heavy Maintenance Expertise (level 1,to 5): Gives airline lower overall maintenance cost
-Training Expert: lower staff and fleet commonality cost
-No Frills Expert: lowers delay probability with shorter turn around times
- ..
Mike