@Riot: add GOOD behavior as a matchmaking parameter
Riot, you've said in the past that, in order to community behvaior, you want to give positive reinforcement instead of just punishments. That's the idea (I think) behind the free skins during the season for people who don't get any warnings or other negative actions against their account.
Why not just make this same "good behavior" an additional secondary matchmaking parameter? If you have fifty or sixty people in queue at the same MMR, try to match those who have cleaner records are paired together. If you can't, or the queue times would be too long, it works just like normal. If you can, then they have a more pleasant experience. I've thought to myself a lot of times "I wish there was there was a tier for people who aren't good but at the same time aren't jerks." I don't see why that can't happen.