Hi there, to try and answer some of your questions, many of your suggestions have been discussed partly in different threads, but many of them may have little or no input from the devs (they read everything, but dont always post). This means that most of the discussions are just within the community
Negotiating with airlines is something that has been discussed, most recently in the thread linked above. But run a search for “Negotiation”, and you will find atleast 6 threads where this was discussed.
For assigning check in desks to airlines, a recent baggage system overhaul meant that you could assign specific desks to specific flights.
This did cause quite a discussion on assigning check in desks to airlines vs individual flights. More here (comment 26 to 36)
As for assigning gates to airlines, each flight is scheduled by you (as CEO), and with the recent change, flight scheduling is now to specific gates (vs randomly chosen by path finding algorithm) via new flight planner. So now, you can make your own unwritten rule and schedule all flights of an airline to the same gate. (See Maple Airlines below)
Regarding advertisements, Olof gave a reply in another thread (not necessarily airline, but private sponsors also)