Un-popular opinion but I personally hate keystone masteries.
They have always been, to me, an un- necessary addition to the game, that just adds to the amount of power creep thats been going on lately. I don't know about you, but I never found it very enjoyable to be killed by a gangplank ult that only did 40 damage to you, but proced his thunderlords for 200 damage. or being killed by a jhin Q that did 300 damage to you and another 350 damage from deathfires. I think without them the game would feel more balanced once again. I know that keystones are very unhealthy on some champions, for example, syndra who hits R on you and then her Ult does like 500 damage and then you take another 300 from thunderlords. I think masteries have too much of an impact now.