I feel with how many spells we can learn from the original schools the next Mastery Amulets we get either are released as Deul MAstery Amulets or add sockets to every MAstery Amulet in game and add Mastery Jewels. If Mastery Jewels become a thing I would suggest 1 way to get them is farm up the LVL 60, 90 and LVL 110 Mastery amulets and through Crafting combine them into a Mastery Jewel that would include anything those 3 MAstery Amulets would give.

I would even be ok if the MAstery Jewels would only be active in PVE content not in PVP to make them fair. I feel Mastery Jewels would also open up more Builds by being able to use 2 other schools other then your own.