I think apart from vanilla classic, other classics should be implemented as part of timewalking instead, including their respective rewards systems. Timewalking rotation through entire expansions with the entire expansion features including the associated class abilities & talents, in other words a simulation of each expansion inside retail where you could go back to Legion Mage Tower and earn those rewards again for example.
They have the tech to do it, but I'm guessing the challenge is allowing multiple class skills/talents/ability-bars systems at the same time. I don't think releasing more classics is manageable in the long run. WoW would need a terabyte of disk space for all expansions to run separately from each other.