My 2 cents: Riot should implement immediate punishments for reportable behaviour. Keep the ban system as is, but if you just have one bad game you should get maybe low priority queue and a box saying "you were reported for toxicity last game and the system agrees. Now you know, use the longer queues in your next 3 games to think about how you can improve your behaviour" or something like that. Explanation and a tangible annoyance towards you. Just something minor. Of course this should extend beyond toxicity, this is just the example I Have given.
Edit: The thought behind this would be that you don't feel like you get away with stuff, because you space out your bad behaviour enough.