Chat
Communication is not required to play or win the game. This makes communication a feature and not a core game element.
As a game feature, I suggest that text communications should be an opt-in game feature.
Voice communication is an opt-in game feature. Voice communication is better than text communication at helping the player play and win the game. That is the reason all the professional teams use voice communication instead of text communication.
Opt-in chat is the only true preventative measure of toxicity. With opt-in chat, Riot could guarantee their users never experience negative chat.
Significantly less negative chat means significantly fewer reports. Time that can be spent on other adventures.
There is a lot of social pressure for game developers to provide acceptable communication experiences in their games. Public good will. Positive public relations.
I have some ideas for implementation too! What a weird idea.