This news comes from Greg Street, Riot Game's VP of IP and Entertainment. In response to a user's tweet, Street mentions that the upcoming game being developed is an MMO.
As you can see in his Tweet, Greg Street's current project is the kick-off of a big game. Although "MMO" may be subjective to many possible genres, it may be safe to say that the new game is an MMORPG.
Riot Games already has multiple "Massive Multiplayer Online" games, but they do not have a game within the MMORPG genre. However, we do not have much information about the new game. In fact, this tweet is the only information that we currently have.
I hope it is an MMORPG. One such game within the League universe would surely be a hit!