Riot has been getting around that for years already. You don't need to change the lore, just explain to people that the game League is not the same as the lore league. Basically, a lot of characters in the game are not actually part of the League, they're just significant characters within the world. There's nothing wrong with that, it's just that they've never actually explained that to people.
Rengar, pretty much any of the void champions, Nami, Lissandra, various others; all of them have pretty much 0 reason to be in the league, but they're included anyway because they're significant characters in the setting.
Rengar, pretty much any of the void champions, Nami, Lissandra, various others; all of them have pretty much 0 reason to be in the league, but they're included anyway because they're significant characters in the setting.