An MMO in a huge world set in League of Legends universe would be awesome imo. Pretty sure I'm not the only one thinking this way (or is that the actual plan?). Most of the characters have some really intriguing background stories--where they came from, their origins, their relationship with other champs--those could be expanded and told in the form of a huge MMO. The cities would be interesting places to explore as well, with Noxus and Demacia war being one of the biggest conflicts going on in there.
Just a thought...