A neat idea for further mastery levels: Custom cursors!
You know how you hit mastery level 10 and you're all "Man, I wish I had something really cool that I could see when I play that champion, or really just in general whenever I want to play because of all the hard work I put into it!"
Well, look no further! I'm here to suggest to you: Custom cursor graphics! While I'm garbage at drawing, I couldn't help but notice that this looks, well, lackluster at best:
https://gyazo.com/0cbab45deaab333174cf183585bc407f.jpg
Now, imagine reaching level 6 with Aatrox. Instead of that cursor,perhaps it would be a pixelated version of his pointing hand? The attack icon would change to his sword. For Akali? Her hand, and her Kama. For spellcasters, how about an image of a spell or something similar? Perhaps it could change to items, Lich Bane sword for mages, Trinity Force sword for AD champions!
With the proper resources and perhaps better cursor ideas, I think it'd be really cool to have the option to use or not use custom cursors like that after attaining a high enough mastery level. It wouldn't be hidden just for the user, either. Streamers can show off their awesome new cursors too!