Size matters, even when it comes to planes: the more voluminous the aircraft the better they fly, with more stability and efficiency (think of the recent, huge Airbus A380). Nothing prevents us from imagining, therefore, a timeline in which we will aim to create ever larger aircraft, to the point of launching real flying airports, able to host and make other planes land on us.
This is the underlying reason for the 'Airborne Metro' concept: it is, in fact, a system similar to that of a bus. You could take a plane, land on a flying airport and leave for another destination: clean cut of emissions, clean cut of noise in airports, clean cut of overcrowding. 3000 passengers at a time is no joke.
The obvious problem in the management of similar aircraft is still represented by the unthinkable cost in making them land or take off: it is clear that something similar to nuclear propulsion would be needed to allow them to fly without ever stopping, or almost. Fleets of giant airports could cover precise circular 'routes', Africa-South America, or Europe-North America: the speculation, developed by a group of aviation experts and technicians, the ACARE (Advisory Council for Aeronautics Research in Europe) is also two accounts in the pockets of these giants of the sky, imagining fuel savings for the aircraft-carriers that should reach flying airports from time to time ranging from 40% to 80% in the case of ocean crossings.
Needless to say, we are decades away from a technology capable of developing a similar scenario: maybe other more intelligent and feasible ones will emerge, the fact is that if we could manage a similar situation, civil aviation as we know it would be totally revolutionized.
Advisory Council for Aeronautics Research in Europe (ACARE)