I'll be honest, I haven't played in quite a while from loss of interest. Early on, it's pretty fun - you find under-served routes, make money, expand, make your way up to more efficient planes, etc. But at a certain point, it just becomes a game of undercutting and competing against other airline networks in a static world. Your planes and operating costs are all generally the same as other players, you share the same geography, etc - there needs to be some kind of evolving environment for long-term strategy and growth. Some off-the-cuff suggestions:
- Growth of airport size, depending on traffic
- Increasing attractors for business/tourism
- Population changes in income and numbers
- Random events... like an epidemic or change in relations that lead to changes in traffic