So, I've seen the tweet asking if we want a League MMO.
I believe the best way to know if players actually want it would be a kind of poll in the client, since not everyone uses Twitter or follows people like him.
I for myself would really love it, just to actually experience the story and not just gather it from tons of scattered information and random loading screen messages.
Also, if Riot Games decides not to work on an MMO, maybe think about a story mode in League, similar to the Star Guardian event gamemode for example.
The same way you got the co-op story mode Save the world and the separate battle royale in Fortnite.