Mastery ranks should degrade after a certain period of time of not playing a champ.

For example: I'm currently mastery lvl 4 on Fiora, however, I don't remember the last time I played her. Also, many people say that with Lee Sin, you have to constantly play him in order to play him at an exceptional level, or you will get rusty on him.

So, it would be wise to see these levels start to slowly drop based on what level you have them at.

Say, you have a champ at mastery level 1 only. Well, perhaps after about 2 weeks of not playing that champ, that mastery drops to lvl 0. Then, with every other lvl higher, you increase the amount of time it takes of not playing that champ in order for it to drop to the lower level. Say, a month for lvl 2, 2 months for lvl 3, and so on.

If you happen to have lvl 6 or 7, then it should take well over a year or more for those to drop, because you have put ALOT of time into those champs. You may not play them for maybe a month or so, simply due to meta, or some balance shifts that may have put your champ in the gutter, but chances are you'll go back to that champ after awhile. However, if you don't play that champ for well over a year, chances are you've moved to a role that that champ doesn't belong in, and you happen to stay in and main that role (say a Yorick main picks up an ADC, and stays there).

However, since it would feel bad to lose that prestigious lvl 7, it should be quite easy to get it back if you so choose. Currently, it takes 2 lvl 6 tokens to get lvl 6, and 3 lvl 7 token to get 7. If you were to lose those levels from time not being played, you would only need 1 lvl 6 token, and 2 lvl 7 tokens.

But, yeah, just an idea I had. Feel free to share your thoughts.