Dynamic city based demand + connecting pax = ftw 
Agreed, but I think we are missing vital and somewhat easy step as well. I am not entire sure if we have discussed including "fantasy" events? I haven't been following the thread for the last couple of months, though I just caught up on this last page.
What if we make the game worlds into a more alternative reality? For example, a war breaks out between 2 previous friendly countries with high pax demand between eachother, thus eliminating demand for those routes. I think there could a canned amount of events that could happen randomly throughout the 30 years of the world. This would change the game from mimicking real routes into a game that could be dynamic enough to provide significant challenges for older, less flexible airlines.
To modify Curse's quote...
Dynamic city based demand + connecting pax + dynamic events= ftw
Some possible events include:
-High Speed Rail opens in California, connecting Los Angeles and San Francisco- LAX-SFO pax demand in half.
-A Tsunami hits a country, drastically affecting flights in the affect region.
-The Vietnam War never happens, or happens at a random time.
-World Economy changes, thus passenger demand goes up and down significantly in a few short months (e.g. our current recession)
-European Union initiation date is random, or potentially nonexistent.
-Any country or region could have a major oil discovery, dramatically improving their economies and passenger demands, along with decreasing the fuel price.
-With any war, there could be an associated rise in fuel prices.
-A Country may gain its independence randomly, if at all.
-Diseases, natural disasters, volcanic eruptions, etc.. The list goes on.
I think you get my point. These could happen once every 10 years, or 2 with in a few months- it would be completely random. I don't mean to include ridiculous, unrealistic events; but having these events affect the game world's alternative reality would drastically improve the excitement and challenge of the game. For some events, (like building the HS rail in California) an announcement could be posted saying that construction has begun on the project and it is expected to be finished in the late part of 1999. This would give airlines notice and they could then adjust to the coming changes. Other events could be completely sudden and unexpected- natural disasters for example.
It would all be random, thus making the game quite dynamic and exciting. And because you couldn't necessarily anticipate things happening, it would be harder to plan farther out into the future. These events would also improve the gameplay for old, established airlines because they could have hard times adjusting to these events, leaving "holes" for new startups or simply bankrupting them completely.
What do you think?
EDIT: I forgot to say fantastic work to Sami and everyone else. This concept sounds like it is quickly becoming more than a concept and I am quite excited to see it in action!