I am guessing one of the Wizard subclasses is going to be something other than one of the eight schools of magic that the spells are typed by. One of the difficulties in getting new Wizard subclasses over the 70 percent threshold was the slight drag caused by the identity of the class and subclasses being so tied to the eight schools. That would be like having a Mage class with subclasses called "Red", "Orange", "Yellow", "Green", "Blue", "Indigo" and "Violet". When you suddenly try to add a subclass called "Metal" or "Vivomancer," it just seems off. The PHB is a great place to break that symmetry before people start mentally organizing the class that way. In fact, I kind of wish that we got 4 new subclasses and delegate the spell school choice to be a one level feature that you get at first level which might just be the scribing feature and a couple of bonus spells with the subclasses more about the role you play in adventuring like War Mage, Scholar Mage, etc.