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I watch Guardsman Bob stream. Time to time, he will mention that Riot should remove tribunal reporting and add the ability to "upvote" players with whom you enjoyed playing. He always mentions Reddit as a good example of a self-moderating community.
What he is advocating is a recommendation engine, a la Netflix or Amazon. I think it should be factored into matchmaking. Such engines can be difficult to implement, but I think even the simplest algorithms will make a significant difference: