Allow a certain amount of slot allocations in the first year, and gradually increasing as it goes along, if your based at an airport -get a tripple slot quota, but outside airlines get a single or even better -half quota, per annum.
An airline cannot hog most of the slots, as in this game, it must be reasonable distributed, maybe sell the slots seperatly, as opposed to selling them with the flight creation!!!
Also, I agree with the crazy out of control growth, maybe max 5 aircraft in first year, with max 20 in 3 years and so on!!!! - this may require longer games- but so be it.
I personally think, most of the posts I read here, they want more reality, they dont want to hear this is a game, this is there own creation and their baby, so maybe rectify some things....