We are also passed the experimental phase of the playtest.Currently, it's a playtest and we haven't gotten any refinement on the mastery system.
Other than "people really liked it", they haven't gone though the feedback.
WOTC isn't going to playtest "Build a Weapon". There isn't enough time to test it. They wasted that time in the OGL fiasco.
So unless Mastery gets very very bad survey results, this is what we are getting and it will just be tweaked.
If we are lucky, we might get a few new core masteries if there are enough "We want more masteries" or "Too many weapons have Flex".
But there isn't enough time to playtest a "Build a Weapon" well by the time WOTC needs the PHB to be done to send to the printers next year,
Exotic Weapons in aDM or splat book could add new masteries and help DMs flavor their regions and monsters with new stuff.
Orcs charging the party with double-axes, cleavers, and waraxes could give a standard orc enounter a bit more flavor and new goodies for the party if they survive.