No. Cross-media tie-ins rarely do well under the best of circumstances. League of Legends is a decent game, but the setting is not exactly rock-solid. That's not a problem for the game, because it allows for later expansion, but for a cartoon series it could create issues. And that's not counting the inevitable ****-storm that will occurr when one champ gets passed up as the protagonist in favor of another. A single short film would be cool, but we've already got one or two of those, and Riot has shown that they prefer to render those in full 3D rather than as a 2D cartoon, so there goes that.
Could it be done? Oh absolutely. Heck, Riot could make the entire thing in-house in Adobe Flash if they needed to. (For an example of a professional-quality cartoon made entirely in Flash, check out Wakfu. Warning: It's in French, and the English dubs are atrocious. Subs can be found if one digs deep enough.).
It would probably look a heck of a lot like the game, only with one dimension missing (here I refer you once again to Wakfu).
I'm confused. What exactly would it be about apart from the lore? They'd have to be thick as several pallets of bricks to bring any parts of the game mechanics or metagame into it, unless they buried it all in great hills of lore. Even if it were just a five-minute fight scene it would technically be about the lore, unless they had life bars over everyone's heads.
I'm again confused. If Riot were to do/outsource this, they wouldn't come down to the Lore forum and ask if anyone here knows their graphic symbols from their actionscripts. They'd just make the **** thing. Barring getting a job at Riot or whatever studio they contract to do the job, no single community member would have an impact.