Spice it up, Icons:)
Im very happy with how the mastery system is panning out, but what if we could earn more icons in the game for champions, and achievements within the game, like earning a certain amount of pentakills, and each tier gives a better icon. and just in generally have an achievement book that you can fill out with monster/minnions kills turrets destroyed, each with many different tiers to keep players constantly trying for a higher level, keeps it more interesting i guess and even if you loose a game at least you can still feel accomplished :) just a thought.