Shadowedeyes
Adventurer
As mentioned, there are already ways for classes without native access to obtain weapon masteries, either through feat or multiclassing. That may be a tough decision taking up an ASI for a casting stat, or slowing down new spell levels, but thems the breaks. New or revised subclasses might eventually also allow access at some point.
Monks already have some weapon mastery like effects, mostly in subclasses like Empty Fist and Elemental one, and casters of course have a bunch of spells and cantrips that can duplicate a lot of the weapon mastery ability effects.
Of course, if everyone in the game is down for it, you can do whatever you want, including the proposal. I don't think casters need any sort of boost, even minor ones, and I personally am not a fan of diluting the supposed weapon prowess of the fighter to everyone who touches a weapon. Heck, I'd probably have made weapon mastery a fighter only ability if I had been in charge.
Monks already have some weapon mastery like effects, mostly in subclasses like Empty Fist and Elemental one, and casters of course have a bunch of spells and cantrips that can duplicate a lot of the weapon mastery ability effects.
Of course, if everyone in the game is down for it, you can do whatever you want, including the proposal. I don't think casters need any sort of boost, even minor ones, and I personally am not a fan of diluting the supposed weapon prowess of the fighter to everyone who touches a weapon. Heck, I'd probably have made weapon mastery a fighter only ability if I had been in charge.