I could have lived with weapons getting their own assigned weapon masteries, but I think that taking the versatile one that most people hated and then just giving them all sap was a poor decision. I do think that it should have been you learn how to use a specific weapon mastery and could use it with any of the appropriate weapons instead of learning specific weapons. So you could learn how to topple with any of the weapons that had access to the topple mastery and not just have the quarterstaff mastery.
it could be open or it could be per weapon.
I.E:
If I want only to use shortswords for 2WF and I have 2 mastery slots(as rogue)
I could learn, Vex(shortsword) and Nick(Shortsword).
then I can use Vex on my main hand attack and Nick on off hand attack, or if there is a round where my Bonus action is free I can take offhand attack as Bonus action and use Vex on both attacks to keep the advantage train going.
or if it's open completely, I would know Vex and Nick and use it on any weapon that applies.
Vex on every Ranged, Finesse or Light weapon
Nick on every Light weapon.
Off topic, STR based weapons need their damage bumped up a die category.
Rapier and Longsword cannot have same damage die.
Rapier uses two possible abilities and Dex can be used for ranged weapons.
therefore, Rapier d8 should be equal to Longsword d10 as Longsword has less utility.
and if Longsword gets d10(Versatile d12), maybe the UA hated Flex mastery would not be that bad as it would be d12 no brainer 1Handed attack based on STR vs d8 with Vex attack based on DEX.