Yeah I too thought about a League of Legends campaign and honestly I thought about it 2-3 days in when I first played the game. I was never a person who cared much about "multiplayer" games I'v always been more into RPG games or just single player games that I can take my time playing etc. Reasons mainly because relying on random people and always thinking someone is cheating etc etc has always annoyed me. Bad example but a reason is because of Call of Duty MW 1. I loved that game but after 3-4 months everyone started to cheat on it and same with MW 2 until I knew that multiplayer games just aren't for me.
I always thought about Vi/Cait/Jinx as like a Crackdown game/concept when I first played this game or maybe something like Batman Akham Asylum style.
Master Yi/Riven/Yasuo as a Ninja Gaiden type of game
But I thought what character for the most part should the player play as if League was an "open world" and I thought maybe Ezreal; I mean he has range and with a different type of path maybe he also has a melee attack maybe a sword( concept came from Final Fantasy/Cloud but maybe it's too close to comparisons idk. Another would be Garen for the brute/tank class players like in Skyrim for exampl.
For me it's not really a campaign but I'm still asking/waiting for a "tv. Show" nothing $ like Game of Thrones or Walking Dead but something that just isn't 1. LCS 2. Playing the game 3. Watching twitch of someone playing the game.
If you ever watched the DBZ Team Four Star channel they parody DBZ but they seem legit and I wouldn't mind watching something that makes fun of certain characters and lore from league.