I feel rather than implementing suggestions such as having upper limit on a particular plane etc. it makes more sense that there be realistic maintenance related options and safe flying of planes tied to such maintenance schedules or say seat demand tied to such age so that it automatically ensures that players don't just keep on expanding fleet but pay proper attention to their revenue/ profit stream.
Also new planes scheduling can become lot more real market oriented wherein each plane has fixed limit as to how many new planes can be brought into service and price each player pays to buy a plane be dependent on how many he buys in an order, his delivery schedule (where one has to pay premium for booking more than fair share of new plane production capacity). This way new plane purchase also will necessitate strategic planning. Similar model can be implemented for leasing a plane as well.
This will also make plater strategically plan what kind of fleet he needs and his network accordingly than mindless expansion.
Second option that can be added could be adding seasonal demand/ fares schedule so no one is sure of demand all the time. This, when complimented with short term discounts etc, will cause players to also have more flexible schedules and make each airline dependent on market demand/ supply fluctuations like real network.
Thirdly one can develop alliance sharing features like code share etc. which will cause players to co-operate and develop more coherent network. All this will eventually lead to one having more realistic fleet and will also add planning aspect to the game.