Justin Kranzl confirmed to Variety that Marc "Tryndamere" Merril's tweet asking players whether Riot Games should make a League of Legends themed MMORPG, following map release, turned out to be a joke, even though many hoped otherwise.
Our hearts and feelings will remain forever unfulfilled as Marc "Tryndamere" Merrill relayed the sad news that he was not being serious about a League of Legends MMO. Justin Kranzl, Riot Games' North American communications director, stated that Merrill was only "being playful around the release of the world map".
Another intention of Merrill's was to encourage the players "to think of what Riot's long term goals could look like", which a double edged sword for the fans. On one hand, it evidently means League of Legends MMO has not yet been conceived but on the other hand it means that it could be in the future. Either way, it's not coming any time soon.
Considering an MMO development cycle normally lasts about five years, it also means we will be well into the next decade, should Riot Games actually decide to pursue such a course of action.
It is somewhat odd that Riot haven't actually started working on an MMO yet though, as League of Legends has built up massive lore that could serve as the groundwork and they have Greg Street in their employ, who is well known around the world for his work on World of Warcraft.
On top of that, League of Legends is already massively popular in China and Korea and both regions are known for their love of MMORPGs. Therefore, it's easy to figure that releasing a League MMO would be a slam dunk on popularity all over the world, and especially in China and Korea.
Riot Games don't seem to be understaffed either as wikipedia page lists them as having around 2500 employees, although it is unknown how many of them are actual developers. In any case, we can still hope for a distant MMO release, but it's probably so far into the future that Cyberpunk 2077 will release years ahead of it.