There is also a game called "Airbucks" that is a business simulator. This might give you some ideas. Also, "Transport Tycoon " is another simulator that will give you good ideas about how to do growth.
Can I play devils advocate and ask a couple of questions?
How are you growing the population of the cities?
These economic indicators, along with population growth for other reasons (such as a new industry opening, football stadium, etc..), could be used for the basis of your demand.
Are you planning to have new airports built through the duration of the simulation?
For demand fluctuations and market preferences, you can use a series of quality of service indicators (QSI's) to determine market preference. These are a calibrated rating system on passenger preferences that can be used for many factors.
A couple of examples are:
Aircraft - an A320 might have a rating of 1.0, a B747 might have a rating 2.0 and a Fokker F50 might be 0.3 etc…
Sectors: A direct service may be 1.0, one-stop 0.8, two stops 0.4 etc…
Brands: Brand A = 1.2 , Brand B = 0.9 , Brand C = 1.1, Brand D = 0.7 , etc…
Brand preference is based on factors such as local market presence, advertising, safety record, on-time reliability, frequency and long-term pricing policy.
Pricing can have a huge influence on demand, referred to as price elasticity. This basically says that demand is stimulated to a certain extent by the fare class. To dramatise, if you have a fare of $0.00 then your demand is probably the entire population of the city-pair. Even at $10.00 you start to create a financial burden which may eliminate 20% of your local population from flying.
As you get closer to the accepted average fare in real life, fluctuations are more measurable. For instance, a widely accepted equation is that a 1% drop in average fare will stimulate 2% growth in traffic. This works both ways but of course, is not as linear as that (I do not know the exact algorithms, or even if there exists such an algorithm). Don't forget cargo either.
If possible, generate your base demand on origin and destination (O/D). So for instance if you start the simulation back in the early 50's, you may have a demand for a market like New York to London. At this point you have to go via the Azores or Greenland, but as aircraft range increases, you can go direct. The QSI's will determine that the company that offers the direct service is more likely to get the demand. Subsequently, the demand for Azores or Greenland should fall as the true demand is being catered over the direct sector (should also increase the brand rating of the start-up carrier).
I hope I have given you some ideas. Please put some specific questions down if you want more assistance and I will see what I can do.