Mastery 8 (or higher)
I think they should add a new mastery level or levels
After you do the quality but generic set up for getting up to level 7, i think the next level (or levels) should be more specific.
This would be in the form of 3(or pick a number) of quests for example for azir mastery 8 or mastery 8, 9, and 10 the quests might be:
get 3 pentakills or get 2 pentakills in the same game interrupt 2 movement abilities by enemies at the same time steal 5 dragons/barons/rift heralds