In real life, airlines often have several aircraft that are spares or owned just for parts. I'm NOT suggesting we implement spare aircraft. That would involve implementing maintenance delays and cancellations, and it would require players to actively manage their airline to swap flights over to the spare aircraft.
What I AM suggesting, though, is that we can have "hangar queens". Aircraft that we can allow maintenance to take parts from, etc. I think it could be a simple action of checking a box under fleet management. "Use for maintenance", as an example. If you have an aircraft designated for maintenance, you wouldn't have to have seats or pilots for it. Age wouldn't be a factor into ORS or image. It wouldn't fly any flights. What would be the benefit of doing this? I suggest an improvement on maintenance price, quality, efficiency, or any combination of the above. If you have more "hangar queens", the better the benefit, but I do think there should be some sort of a cap so you don't get free maintenance.