Add an explanation for the grade you received?
I kind of dig the new Mastery system, and the grades at the end of matches are interesting. Could we perhaps have a page explaining why we received the grade we did, maybe as a point of reference to what we could improve upon for that champion/role?
Perhaps break the grade down as a percentage based on wards, cs, KDA and so forth, based upon how the grade is derived from similar champs played in that role. Probably couldn't fit it on the end of match summary page, but you could add a tab to the match history site. I think it might add some insight into Riot's grading scale, and maybe encourage a few people to focus on areas to improve.
And, you know, ward more. 