New mastery system rewards suggestion
Good morning!
I know you said that skins will most likely not be part of the mastery system rewards. However, I believe it would be a huge motivation if a "simple skin", like for example Riot Girl Tristana or the Blitzcrank chrome pack red version which is not a super special one but a rather simple alteration, could be the reward for mastering a champion to the last level/tier (once you added the further tiers you were talking about so that you only get it on the highest level which will take quite a while and effort).
Just an idea to increase the motivation and give one possibility of earning a "basic skin" for your favourite champ (and move away from making skins buy-only (only really "basic" ones, as said)...and thus probably giving a massive boost to that system.
All the best for the week!