I was having a good solid think about this. What if it was related to the contract completion.
Say we’re at INN to begin with and you unlock a local contract to Venice with Austrian. It automatically provides the smallest like it does now which currently is the Q400 but which may in the future be one of the smaller Dash 8’s Dev willing 
. You land it, handle it and send it off and it comes back with the 1/1 contract completion. What if by clicking the contract completion button and then renew the contract it unlocks the next tier for Austrian, the F70.
You can then open a new small menu, say where the current information button is, allowing you to tick the Q400 off if you so wish, the F70 will automatically tick on.
This would give you control over the type of aircraft you want on a route and would allow for larger aircraft to be included, say for BA at LHR allowing for 757s to be available on routes into Europe, and once you hit 10/10 you may unlock the 767 or something. This would also work well for new and old liveries, say for ICE or Condor and the likes.
Again, reverting back to Austrian, you unlock the A319 (Not yet in-game I know 
) and you get the choice of 3 different liveries, all that can be ticked on and off based on player choice (I’d have mine all ticked on as I like the variety), this would then give the player a randomised A319 like one of the 3 below,
So instead of having 3 contracts to have 3 different liveries at your airport, you could just have the one, allowing for more range in regards to contracts and for more variation, both in the liveries operated and in the aircraft types operated. You could even include in this feature winglets, as I know some players don’t like the variation such as on United’s 767’s (Which I love as I remember them rocking none back in the early 2010s).
I addition to that, you could then have special liveries unlocked at say 10/10 which would spawn say 10% of the time on a contract, locked if it is already on the airfield to prevent you have 7 arrive one after another, if you’re so lucky 
.
I’d love to see this looked into, it’s a fantastic suggestion Krankenwart
Also, for an extra bit, Condor would really benefit from this as you could have such a mix of types and liveries coming in and out of your airport,
And ofc you could get this mixed in then,
United as well would be at an advantage especially at IAD. If you wanted only the Continental/United livery, you could tick off the new blue livery and vice versa. Also, as I wrote above, it’d allow players like me to run a Blue Tulip fleet which I’d very much like to see, but understand it would fill up the contract selection screen and if players didn’t want it, would be annoying if they appeared in the random selection.
Actually, even better for the sheer amount of liveries and airlines I’ve requested and how that could be implemented without it potentially being a burden for other players.