Recently I opened a new line from my HQ in Sofia to Palermo. After the mandatory 5-6 weeks to build up airline awareness I went to increase the frequency of the flights, however the airport was not assigning me any new slots despite my awareness to have increased more then sufficiently and despite having 200 slots unused.
At that point I started looking more into the mechanic of it and at the airlines flying from Palermo. It turns out there is only 1 HQ and one base in Palermo. The airline with a base had 11 routes, mostly long haul routes. The airline with the HQ however has 53 which probably accounts for more then 50% of the slots at the airport and as it turns out that airline is an AI airline.
Also because of the mechanics of the game I suspect that both of those airlines hold additional slots that they may never intend to use, due to the mechanics of the game, which automatically assigns slots based on milestones rather then interaction.
*side-note (just as I was writing this additional 11 slots became available over the 200 so I was able to increase the frequency of my line)
This brings up a specific problem that might not be that big or urgent right now, but would become a pain if the player population ever increases which I assume will be the ultimate goal.
There is a need for a means to "eliminate" AI players as the player population grows or an airport becomes busy and a need for an ability to expand airports during the game.
The first part can be handled softly by programing the AI airlines to close any route which gets 2 real player competitors and close shop if they reach fewer then 10 routes. This way they would be seamlessly retreading as the players expand, still serve their purpose and not impede the ability of 0players to compete with each other.
The second part needs more complex solutions and I think it should be tied together with a future "growth" system, for cities and airports in general. That part deserves it's own discussion though and I plan on throwing a few things together if there is an interest at all.