Possible Idea to Reduce Toxicity
NOTICE: This is just a personal idea and is something I thought of, it would be flawed but is a building ground for future improvements.
Every ranked season, before placements, you'd fill out a survey (maybe 50 questions?) that would ask about your play style, competitiveness, likelihood to tilt, and other identifying traits. This would help match you on a team that had like minded summoners and reduce the odds of you being placed with someone who could be too passive or have zero game knowledge and cause you to tilt or flame.