Mastery and what's wrong with it...
At current, the mastery system is a grind and luck (or pay to play). Although the grinding actually has merit; the luck portion is not mastery and doesn't suggest skill on a champion. Malcom Gladwell in "Outliers" suggested mastery of a skill or subject is obtained around 10,000 hours of practice (he used examples like the Beatles). Which means if we're playing a champion for multiple games for multiple hours we're probably getting better at that champion and regardless of innate talent or understanding of the game will begin to get an understanding of that champion. S's (and other letter's) are a statistical representation of how other people perform with the same champion at various levels of play. As with the nature of all statistics, to obtain these "ranks" are merely a matter of # of games played. This grind and getting the S's does have a bit of merit.
Luck in loot, as it pertains to mastery or paying for that luck, is irrelevant to performing well on that champion. There's simply nothing in terms of skill or experience that has been proven to suggest an increased level of mastery of that champion for 6+ ranks. I could go on, but there's no point. The 6+ mastery part of the game is a failure in terms of how it relates to player performance. If it's increasing profits for riot... good for you. But it's a misnomer at best and a blatant failure to understand the definition of a word at worst.
Many games have ways of testing the skills of their players through achievements and quests. Although I don't think we should have quests like "steal the red buff from the enemy jungle" because that would promote poor gameplay or be easily manipulated there's an opportunity here that would result in better gameplay.
Examples: Die less than 5 times in a winning Game with a positive KDA Get 10 assists or more while team fighting (4 or more players around) in a single game. As a support get 200 assists on the same champion (winning games counted only) Dunkmaster - As Darius, use r to kill at least 3 people within 60 seconds Sniper - Kill the dragon or baron without any members of your team being inside the lagoons. Tower Defender - kill 100 champions underneath a tower while you're outnumbered Tower Crusher - Take out 100 towers without taking damage from them. Creepy - Obtain the last hit on 25 minions in a row. Creepier - Obtain the last hit on every minion in the first 5 waves. Creepiest - Last hit 1000 minions. And so on and so on...
These achievements or quests could be specific to champions and award 1 point after completing 5 quests per champion or account based and just add to the total mastery score. There could be a point system but the idea is that there are actually things in the game that make your champion better or you better as a player. The goal for mastery in my opinion would be to highlight that players can prove they're capable of those things. Can you be a good blitz and miss over half your q's... sure. But masters are going to hit them every time and have probably wombo combo'd with thresh a bit. The point is simple. Mastery suggests that there has been sufficient proof of skill. If there system doesn't provide that then it doesn't work. At current, it doesn't provide that and doesn't work. Is this the only solution? No. It's just one of many that I hope we're working on.