My worst experience in league is still from when I was lvl 7ish
Long story short, I fell in love with Aram as soon as I tried it shortly after the bot grind in early season 4.
I barely had any experience queuing alone vs real players, and because I didn't know what most of the champs I got did I didn't care about trading but after a few unsuccessful attempts I decided I want a strong champ that I could manage, then I got twitch.
I was really happy but someone said something like "yo I'm a beast with twitch let me have him", to which I replied, "Sry I'm new and need a strong champ to do good".
Then they said some really awful things. Things that today I have seen literally thousands of times(regretfully some of my own words) and I've been desensitized multiple times over.
But to this day, the severity and surprise of the flame I got has left a permanent scar, and really upsets me when I think about it even to this day. It sucked the fun out of that game and really put a dent in my growing interest in league.
I guess my point is I think the solution may sound odd at first but it would definitely be easy and greatly helpful.
By revamping the tutorials to include advice on interacting with other players in the game.
Right click to basic attack, Q to pew pew, that's all fine and dandy but what a player really needs to know is how incredibly toxic his own allies can be, and how to recognize and deal with this unfathomably common problem via in-game tutorial.
Been waitin a while to get that off my chest.
"Same."
mentor support program needs to happen
build 
mute 