I have a small suggestion that I think needs addressing.
When the game starts in 1950, a couple of interesting airports are not open yet. Like Stockholm Arlanda, both major Moscow airports and if I'm right some airports in Saudi Arabia. Because there is no alternative, you can't move to those automatically so the only way to HQ there, is to start the game later on when they open. Which puts you in a (slight) disadvantage to somebody opening a base at those airports.
There is one larger Moscow Airport open in that time (IIRC: DME, and VNO is open all the time), and Stockholm Bromma is also open during the early years, which woudl allow you to start in those cities. I do also think, that just like with HND or CGH, you HQ gets moved, once the new airport opens (otherwise it could be implemented for those airports).
Still doesn't fix the Saudia Arabia issue, because there is not a single airport before JED/RUH in the late 80s or early 90s. But such is life, There was no need for air traffic there, back then, so why should it in AWS?
2. Maybe to make things more close to reality, there could be limited places in early game years with limited profit, so after a year open more and more slots for players therefor it could be quite interesting to join the game not only in 90's after collapse of USSR, but even after that
3. There should be option for more than one stopover, for ex. BOAC flight from Australia to London with first passenger jet Comet. There was 3 or even 4 stopovers during flights.
@2: Player number is limited at first, and increases by 1 pre game month until the end of the game (or shortly before end).
@3: you can have up to 2 stopovers per direction. These are not used for Passenger de-/boarding anymore (was changed maybe 3 or 4 yrs ago). Flights with 1 tech-stop are already hardly worth it, adding more stops would send your airline to BK, or at least result in major expenses. Be happy the system doesn't allow you to ruin your airline in that way
(plenty of other ways to kill your own airline though