I prefer the exploit to be tied to the weapon's characteristics, with Fighters being able to modify the characteristics because they're better at Fighting than anyone else. This feels like something that's explicitly Martial but also something to be shared between Fighters, Barbarians, Monks, Rogues, Paladins, and Rangers (and some martial-gishy subclasses) that shows why they're all part of the Martial Power Source.
I'm glad that the rigid grid of Class Groups were dropped this packet because it's clear that even if Ranger is an Expert it's also a Priest and also a Warrior, and Paladins are both Priests and Warriors, and Monks are Warriors but with substantial Priest and Expert flavours but perhaps not features that are usually shared by those groups… instead we can show these class group thematic ties between classes by shared features like Weapon Mastery, Fighting Styles, Magical Power Source Spell Lists, and Expertise-granting features.