You know, I actually like the Champion Mastery System, But....
I wish there was a bit more too it. The actual leveling is cool. It makes you feel like you're getting somewhere and progressing. Unlocking the little Tag and the Taunt actually provides a sense of achievement. Past Level 5 there isn't anything to do though. The number just goes up and up and up. I understand that it's still "In development" and there are apparently plans to expand upon the system, but you said that about Honor and 2 years later it's still useless.
Some people have said to award Chroma Packs if you achieve Mastery which I personally agree with, but I have another, very simple suggestion.
When you hit Level 5, let us have the Champion Portrait (the one in the bottom left during gameplay) as a Summoner Icon. I mean come on, who wouldn't want to have Teemo as their summoner Icon?