The more I think about it, the more I like this idea. If you get a mastery feature, you choose a mastery instead of a weapon. When you make an attack with a weapon you’re proficient with, you can apply one Mastery property you know to the attack if the weapon meets the requirements for that Mastery property. It would no longer serve the function of making different weapons more distinct from each other, but it would make weapon masteries actually offer tactical options instead of always doing the same thing every round, and it would make having multiple weapon masteries actually beneficial, as the more masteries you can access, the more versatile your attacks would be. And it would completely eliminate the “I found a magic weapon but it’s the wrong kind” effect. Personally I’m not convinced that last point is a positive, but clearly a lot of folks would consider it one, so who am I to disagree.