Riot Games may finally deserve to have that S at the end, as they have trademarked the name Legends of Runeterra, which hints at a new, separate game, set in League of Legends universe. They have not confirmed anything officially yet.
Rumours about a potential MMO game set in League of Legends universe have been circulating for years now and the closest thing to a teaser so far has been Marc Merrill's tweet that sent everyone into a frenzy instigated by curiosity, hype, or both. Another Riot employee stated that this was just a play on words by Merrill, after the introduction of League of Legends interactive map.
The latter may have been a diversion though, since Riot Games have filed for several trademarks regarding something called Legends of Runeterra. These trademarks range from merchandise, over online services and cosplay items to those specifically aimed at "Computer and software development in the field of video games" which could potentially hint at a new title in development by Riot Games.
Considering the vast amount of the trademarks and the wide range of products and services they refer to, it is possible Riot are not far from announcing their next big title after all. There is no confirmation or mention of the game being an MMO, except for the above-linked teaser by Marc Merrill.
Bearing in mind that the product will be called Legends of Runeterra, it is easy to assume it will indeed take place in League of Legends universe, since Runeterra is the name of the planet where the story of League of Legends takes place.
Legends part could refer to the champions from the game or even player created champions, who could be out there forging their own legend.
Since one of the trademarks mentions "Broadcast and streaming of video game play and video game competitions over global communications networks, the Internet, and wireless networks" it is quite possible this project will focus on PvP competitions. It only remains to see whether it will be an MMO, or whether it will see the light of day at all.