Maybe its already in the game and im just blind, but one thing im noticing as my airport gets bigger and bigger im finding myself being overwhelmed with a ton of small planes that arent what i want
Wjat if there was a way to sell gates or routes you didnt want anymore? Even if it was for only like 100w that wouls still free up space for routes you DO want (or even if there was a cancellation fee)
It would really help the late game out
Thoughts?
For know it is not possible.
You can RESET the airport and start over again.
or
You can buy cheapest planes as a DUMMY for the slots.
Just simply do not let the S planes to land 
and see hundreds of stands empty.
The game is not designed to have full airports of planes and if you do not want to have empty slots you can upgrade them from S to M so every stand can be used.
Right, but what if i want a specific route/aircraft type ti show up more, and it doesnt show up because all youre seeing is other airlines, or other aircraft types?
If you do not want to see any airlines then you should not have unlocked their routes. Only the ways is the same - do not let them to land.
Well think about this scenario then
I just unlocked Prague and I noticed that in the achievements I can unlock the ability to purchase a 190 if I land hunter them, so my first thought would be great, Iām going to unlock as many e190 routes as I possibly can so I can get that unlocked as fast as possible
And it works, but then suddenly im stuck with a ton of roytes i no longer need late game,
So, if i could clear out that clutter then id be able to save space in the cue for planes i do want to see