Wouldn't the next logical step be a Challenger-color themed Mastery 8, obtained with 4 S+'s?

make it 10 S+ to make it an actual challenge.

and no blue essence this time. your final mastery emote should also display your mastery score on that champion.

I think there should be a type of super temporary mastery. One that looks incredibly cool in loading screen...And looks super cool when you throw the taunt out in game.

How do you obtain this, Well it's simple. Gain 5 in a row. How do you keep it?, Continue to get S's on that champion. If you fail to get an S, Mastery gone ^_^, well, till you get 5 more S's. lol.

We will call this mastery..... SUPER HOTSTREAK MASTERY.