I recently posted a new idea in the German part of the forum, and I wanted to share the idea also here to get a broader input on it.
Existing games worlds start to become at one point or another by a few big, dominant players that fill most slots on the airports and make it difficult for other players to enter the world.
My idea therefore was, that it should be possible for a player to invest in an airport and be able to build an additional runway. The cost though should be very high, to not just have anybody build runways at will. The runway would also need to be available to all players equally, so nobody gets preferential or exclusive access to it. I would even suggest, that every player gets an in-game mail with a note stating that on airport xy a new runway will be finished in 2 or 3 weeks to allow them to prepare for the event.
Alliances or multiple players could obviously work together to build runways at various airports and thus increase the overall capacity of a given world. As the demand though would not necessarily rise, it would mean you'd have to compete with more players for the same share of the pie.
Of course, it would deviate from the "real" world, though if airports become too congested, they also try (some more, some less successfully) to expand and build either completely new airports (like Turkey, Beijing, etc.), or build additional runways (like LHR).