It should be nice if C-maintenance could also be scheduled, so when you have larger fleet of same types of airplanes, an extra plane can take over the schedule of the plane which need to be in a C-check.
Here (https://www.airwaysim.com/forum/index.php/topic,114.0.html)'s some discussion about this matter. Dates quite back, September of 2008, but is still quite relevant.
Also, it could now be easier to adapt, since fleet commonality has become more important that back then, so the players who really need this feature (big airlines) most likely won't have that many different aircraft of the same size anyways.