The reality is this game is already going to be toxic because it's a MOBA that matches you with 4 random teammates who you will most likely never play with ever again after the game. The game is very popular and runs on a toaster meaning it is going to appeal to a wide variety of people especially little kids. There isn't much that can be done about this.
I can't even imagine actually being a new player to this game. Does this game even retain its new players? If I had to guess I would say no. I tried leveling this new account. It is literally pure cancer. Obviously you are going to get a mix of smurfs and some actual new players. Omg the abuse that these new players have to endure is literally insane. If I were new I would definitely not continue playing this game. Every game someone is a "pussy" " nig ger" or "%%% got." If they aren't saying it in the chat then it's in their clan tag or summoner name itself. Literally had one guy with KKK in his summoner name.
The problem is Riot decided to be the front runner of trying to make a competitive game some online utopia where no one trash talks and they go to great lengths to keep the chat clean. The problem is you give the trolls all the power in this game because you literally get away with anything in this game as long as you don't say anything in chat. I can literally go into a game and just troll over and over. Die 15 times a game. Do some troll build. Buy 6 tears of the goddess. Follow my jungler around stealing jungle camps. Guess what? Nothing will ever happen to you as long as you don't flame in chat and the people who troll know this. Riot never deals with many of the root problems of toxicity. They only deal and punish the symptoms. Obviously this doesn't apply to all cases. Decent people eventually become frustrated and lash out. Does it make it okay. No. Does it happen? Defiantly.
Since Riot puts so much emphasis on toxicity the community now thinks everything is toxic. You can no longer even ask your teammates to do anything without them thinking you are criticizing them and being toxic. Everyone immediately becomes defensive about EVERYTHING and if they don't start flaming then they focus the rest of the game on how they are going to report you and spend the other rest of the time trying to get everyone else to report you. All because you ask them if they can toss a ward in a bush or ask them tell them that Graves is going to keep coming top if you keep shoving and don't ward. It's gotten to the point where the chat window is useless and it is best to just mute all your teammates.
All the power is given to trolls since they can waste your time and you can't do anything about it. You are mad because you didn't get as many ganks as you wanted? You got camped because you didn't ward. Guess its time to troll now, but as long as you don't say anything it's all good, but if one person on your team says one thing because they are trying while you waste their time then they get reported and banned. The system favors people who just want to piss other people off. Had one game where we were getting stomped. A duo is fighting with some guy on the team. I'm trying to surrender because this game is 10000% over. Guy literally tells his duo in chat. "Hit no and draw the game out, I want to waste these losers time."
The matchmaking and entire MMR system is one of the biggest failures I have ever seen in online gaming. It has already been confirmed by Riot multiple times that they have algorithms in place to put players on losing streaks in with people on winning streaks. The game tries to hold you to a 50% win rate. Why? If a person plays well and is winning a bunch then you basically punish them by giving them a bunch of players on losing streaks who are typically titled or who are just outright aren't as good and deserve to fall? I believe this is what has created this whole situation where so many games feel like stomps because you have a bunch of players who deserve a higher elo mixed in with a bunch of people who deserve a lower elo creating this giant cesspool of players of various skill levels all the way from bronze-plat. This is why a lot of high elo players think that silver is the same as plat. That statement is actually becoming more and more true thanks to Riot's wonderful algorithms. This giant mixture of players of all different skill levels leads to many frustrating games. More frustration= more toxicity.
Riot also decided to make this game more team orientated meaning that the worst player on each team often has the bigger impact that the best player from each team. This leads to a frustrating experience for people who consistently do well and get constantly matched with goofballs thanks to the system talked about in the above paragraph. To make a long story sort the system does a terrible job at putting people in their real elo or at the very least makes it a huge grind. These leads to a bunch of people playing at the same elo but at drastically different skills levels.
Even from a game play perspective the balance is getting pretty awful. Way to much damage in this game. Marksmen have been broken for a few years now. Riot claims to be working on it. Takes them forever naturally. Meanwhile they release major changes to the jungle nerfing it and making the popular junglers even more popular and further alienating the weaker junglers. I could go on and on but if you search this board for 6 seconds you can find many good threads about all the major problems this game currently has with balance.
I could go on and on but I believe the major problems are the "report" culture Riot created along with their horrid matchmaking system/elo system along with their even increasing terrible design choices such as the scuttle crab changes.