I fully support ET's request, and have made it a few times, myself, over the years.
The beauty of missions is that you set everything up for all the participants. As the mission leader, you're not constantly ( and I do mean CONSTANTLY ) having to repeat the field, loadout, runway, skin, launch time.. etc ad nausem, for the insanely numerous requests ( even if you type it in text!!! )

It truly is frustrating having to repeat the same thing over and over.
Unfortunately, and I cannot understand this priority, but I accept it, there are those that despise the default skin of missions and will not join missions for that very reason. Also, if you launch in a mission with the default skin, you have to go reset that skin in the hangar the next time you fly that plane. ( another particularly annoying PITA aspect of missions )
I understand there are those who do not load/desire to have extra skins on their machine, so forcing people to use them to be in the mission may not be the result you desire..
With that in mind, I propose that a skin can be set as 'optional' in the mission builder, thus giving the builder the option of allowing non-specified skins in the mission.

Another change, to go with this, is that the mission launcher should NEVER change the skin that is set in the hangar for a particular plane.