what do you think about this?
Consider each possible feature of a contract with an airline as a discrete element. Runway pricing, terminal fee, services, and so forth. Each one can contribute a certain number of “points” which will be invisible to the player.
Adjusting pricing on items can increase or decrease the number of points in the pot. Cut them a break on pricing and gain some points; start gouging and you lose some points. Offer certain services or priority attention and gain some points. Have certain features at your airport (total area of first class lounge available, amenities, the like) and gain points, don’t have them and lose points. Accede to special requirements from the airline - contiguous gates, for example - and you gain a bunch of points, decline and you stand to lose a bunch.
The points reflect the airline’s disposition toward what you’re offering, but as I said earlier, they’re invisble to the player. Under the hood, the game is comparing points on the table to a minimum threshold for the airline to say “yes.” That threshold can slide as well; as your airport gains notoriety, passenger evaluation climbs, and other airlines approve of your work, it can lower the threshold to a degree. The ratio of points to threshold is used to calculate a percentage chance the airline will agree: a percentage-to-accept, or PTA. If the hidden figure of 500 points is the current threshold for the airline, having 460 points on the table will show the player a 92% chance the airline will accept.
Put together the best contract you want (or can) to get the PTA up to where you want it, then submit the proposal. The game waits an appropriate amount of time (a day or two), then quietly rolls the RNG dice, weighted by your PTA and checks to see if you got the contract.
It’s similar in principle to the negotiation mechanic employed in most classical 4X games for “diplomacy” operations, or the traditional Fallout bartering mechanic. Sweeten the deal to improve the odds of acceptance, basically.
Having certain employees or research complete can help refine the PTA - or, if you want to go hard for realism, lacking certain employees or research means you don’t get to see the PTA at all, meaning that you could be handing out proposals that have 140% PTA because you don’t know any better and you’re not making the money you could otherwise.