Positivity and Summoner Spotlight
I was thinking about this at like seven in the morning so if my ideas are trash, blame it on my grogginess.
So I was thinking, as many others thought about as well, maybe there should be a positive feedback thing at the end of every game. It's like reporting, but positive! For example, if some guy was standing up for a teammate who was being harassed, another teammate could right a report on him at the end of the game about how he was standing up for another player and it gets sent to Riot. Or do that teamwork, helpful thing at the end.
And then after that, Riot could hold a Summoner Spotlight every week or two weeks. Riot could pick a few users who have many positive reports and interview them like: How do you keep your positivity in game? What do you enjoy most about League of Legends? What do you have to say for the rest of the community? I know it wouldn't end the toxicity in game, but I believe it would at least encourage people to be more positive rather than toxic.