It would be poopoo and boring and not a real MMORPG, those died out mid 2000's. All you get nowadays are those single player MMO's where you can watch others play the game online by themselves alongside you. Probably lame battle stages that you repeat over and over, the end.
The glorious time of amazing MMORPG's like Ragnarok Online and Tibia are long gone, no developer will put that much time in making a good and complex game when they can just give you a single player mash buttons game that you have to play online to give you the illusion of a multiplayer game.
how would a league MMO even work
To build an MMO, you need characters that are actually likable/relate-able. I don't find any of League's characters relate-able except maybe Ezreal. They're all insane
i'd play it
If it was free.