Bring Back Arcane Blade
I was messing around with some weird AP Ezreal builds and noticed that Nashor's Tooth was doing less damage than I remember. It turned out that the difference was the lack of Arcane Blade. It wasn't really a problem with Nashor's but just a general 5% less AP scaling.
I thought this mastery was pretty nice and encouraged mages to take some risks in order to maximize damage. In the context of Nashor's Tooth, it gave me more of a reason to buy AS for a mostly AP build. When autos scale with more AP, AS makes much more sense and Nashor's Tooth is pretty much the only item that synergizes with AP builds too (compared to Wit's End for example). The lack of Arcande Blade leaves something to be desired thoug and I hope this mastery can be added again somewhere in the Cunning or Ferocity trees.