Hi!
One of the main complaints in the game is that it stops being fun once all slots are gone.
I also understand this is one of the main reason for people getting bored and quitting, and also for old servers being less interesting.
My suggestion is simple, introduce variable landing fees to gradually force airlines that traffic slot blocked larger airports into using larger airplanes. This means they have to substitute several smaller aircraft for one large and thus opening up slots for expansion.
The base landing fee we have now is good as a starting point.
The suggestion is that the final fee is modified depending on slot availability.
0 - 50% slots taken the fee will gradually go down to a minimum of say 25% normal price to promote any airlines flying any kind of airplane (even small ones) to traffic the airport.
50 - 90% slots taken the fee will gradually go back to the standard fee.
90 - 100% slots taken and the fee will gradually increase up to 400% of normal price (or even higher) to force any airlines flying smaller airplanes away and stop them from blocking the busy runways.
I think this also has the advantage of automatically taking care of any exploiting players that are not reported even if they use small airplanes to consciously block slots in a large airport. Once the slots start to run out they are in for some serious financial problems!!!
It also would feel very realistic to have airports that react depending on the situation.
It should be easy to implement since slots available already is displayed. And I don't think it would add much server load at all since you only need to update this once a day (when the airport updates) and it's just a quick check.
% Numbers are only a suggestion, feel free to debate them.