I'd like to make a suggestion regarding the aircraft types used in this game. One appealing aspect of this game is the variety of aircraft you can use. The bad thing is that only about 5 types get used in any large amount. This is because its not worth using anything else.
Some examples are the Q400. Pretty much everyone uses this plane and with good reason. However this is unrealistic. There are not thousands of Q400's being ordered. Same with the 737-900. In reality this is one of the least popular 737 types with the -800 being the plane of choice. I believe one reason the 900 is not as popular in real life is the need for an extra FA. Maybe this could be reflected in the operating cost of the -900.
In the game I almost never see anyone using the 747-400 but in reality it is one of the more efficient planes out there.

My suggestion is to introduce something that will encorage using other planes. This will make the game vastly more interesting and realistic.
Maybe offer older types at a lower purchase cost.

I have been trying to put together an airline based in Australia (my home country). I have used Sydney as my base but I have noticed something. It seams to me that the demand in Australia is far too low. I tried to create flights between Sydney and Newcastle. Despite the fact that I have no competition no matter what price I set I cannot make a profit. I also created a flight between Brisbane and Darwin. With no competition and a price of $49 I had a 0% load factor. In real life both these routes have many flights a week with good loads. QANTAS flies Bris to Darwin and their ticket is $700 one way! Also Sydney to Adelaide. I had a frequency of 4 and my profit went into the red. In real life there is about 20 flights a day between Syd and Adl with an average price of $300. Eveytime I have flown this route the loads have been above 80%.
I think the demand in this area needs to be increased to a realistic level.