I warn you before hand this is an interest check not the actual RP. In Kayle's lore there is mention of one of the High Summoner's named Reginald Ashram he was one of the League's Founding Members and also the reason she joined the League. Strangely without any warning he vanished without a trace this is the story explaining why, it would be long and epic, but what the idea is Runeterra in massive danger, not from the void, nor from the Noxus, no this threat was the real reason The League was founded.
When it's found out why he went missing, it will shock the entire of Valoran.
Anyone interested note knowing of the League of Legends lore would be essential in this scenario or at least reviewing it.
In the game you could play either Summoners or Champions, it's up to you.
This is also an area to brainstorm. :)