Riot, I have to say, it feels rather meh to make it to mastery 7 on a champion. In enough games, its just bound to happen. Having M7 doesn't feel like it is all that rewarding or even a show of experience with a champion. I have played a few different games were excellence was tracked in better ways that showed much "mastery" you had with a certain class/weapon/etc.
One way that I found extremely nice was winrate. Once you reached a certain win rate, you achieved a higher level. If your winrate dropped, you lost it. Honestly, having to spend blue essence to level up my champion mastery kind of sucks. The way im thinking could also increase the playrates and variability of champions in games as well. First taken into account would be your win rate with that champion, say with me with Zed its 52%. It would look overall at all Zed players, and their winrate and determine I am in the top 20%. Good, give me Mastery 6. Those in the top 10% get M7, the top 30% M5 and so on and so forth. Put a minimum on the games played to achieve mastery, say 25. This system would also keep the mastery relative to the players elo, considering matchmaking is pretty fair with placing you in games that are your own skill level. It would be no easier for a bronze to get a high mastery than it would for a gold. Essentially. Here's how the math would work.
First calculate individual player win rate.
Look at win rate across specific champions.
Average win rate = Mastery 5.
Grade on a curve, awarding higher mastery to those with better performance relative to their peers.
Champions that are played less will be easier to get higher mastery easier, therefore more people will play them.
"I want to be unique." (Why I became a Lissandra Main)
If you understand skip this part, if not here's some math.
My winrate with Swain-52%
Average winrate of Swain overall-48%
Total of 33000 Swain players. All over 25 games.
Low End win rate 25%-45% (5000 Players)
Average win rate 46%-52% (21000 Players) Mastery 5 _Also attained by mastery points_
Above Average win rate 53%-58% (5000 Players) _Mastery 6 Cannot get with Mastery Points_
High End win rate 59%+ (2000 players) _Mastery 7 Cannot get with mastery points_
*These numbers are completely made up and just show how the system would work. Not based on actual winrates or the mean of them.