Maybe it would be possible to create something like four different game world lengths.
The first of which is the current length of a Quimby or Otto server, so about 6 to 9 months. This would allow new players to enjoy a new game more often and get a feeling for the game without being overwhelmed by the large airlines you now see at existing game world. It might also be great for players who want to play for a while, but don't want to invest the time that needed for long lasting airlines or know they get bored from the game after several month, a short game world is a better choice then.
The second is a slightly longer game world with a length of 1,5 to 2 years. This would allow airlines to spend more time on creating an airline that can grow bigger and to get more effects from IPOs and also it might enable some airlines to start experimenting with building terminals. It also might have a new impact on Alliances as these might be more important in the long run when the effect of well managed interlining is more effective.
The third duration would be about 3 to 5 years. This is effectively an endless game world in terms of what you can do with your airline. It allows the creation of multiple subsidiaries that can work together, to build your own buildings, to create extensive and well connected alliance route networks and to build the largest airline you can imagine. This would give about all the benefits of the endless game worlds, without the problem of servers that exist indefinitely without much happening in those game worlds (relative to newer servers).
The fourth length for a game world would be endless, but as we already have many of these, I don't think we would need a new one any time soon. This would mostly be as a possible replacement for an existing one when one might eventually be closed due to a lack of paying players on these servers. Maybe you could also add a time limitations to these, but that should be very long (like at least 10 years, or till the server is no longer profitable). This would allow players who want to build an airline to last "forever" to do so, but as most players will probably not stay active players for more then 5 years, this option would, in my perspective, be limited to about 3 to 5 servers (at least fewer than we have now).
At the moment we have 16 servers. With the same amount of servers I would distribute it the following way:
3 short term servers of each 9 months in length. (opening one every three months)
4 medium length servers of each 2 years in duration. (opening one every 6 months)
5 long term servers of each 5 year in duration. (opening one every year)
4 endless servers (opening a new one when an old one is older than 10 years or no longer profitable)