I hate to rain on the parade, but an anime, cartoon, or series in general about the lore behind League is probably not the best of ideas. It might be interesting if done well, but probably not a good idea.
The reason is very simple: the more lore Riot puts in, the closer they get to writing themselves into a corner for champions, maps, items, or anything that could require a lore explanation. For people making an anime/manga/cartoon/series in general, it's not so bad to expand as long as you pace yourself because the show is the only thing you're really working on. Sequels and spin-offs are easy because there's side-characters or members of the main cast you can leave for that purpose.
League is a different story because of champion development and because of the sheer amount of champions they have. With over a hundred so far, and more being added, the problem of creating a champion that feels new or unique grows with every new one they release. That's just from the gameplay front, not even the lore front.
For example: if one of Gnar's new abilities makes him dash to a target point and knock up everyone who's there once he gets there, in the back of your head, part of you goes "oh, it's just a Malphite ult....only it's not an ult anymore." Jinx's ult is another example: it's basically an Ashe arrow, just lacking the stun and adding in some AoE. I could go on, but you get the idea.
Yes, it's nit-picking. Yes, those abilities DO feel different from the things I compared them to. But the point is that Riot already has to walk a tightrope to avoid the 'anything X can do, Y can do better' game with champions, and I think sometimes it puts some constraints on them. Having to fight with that, AND ensure that there's at least a plausible lore explanation and a clean spot for everything to fit into, makes the job even more difficult
Also, frankly, I hate Ezreal. There's far better characters to use. Kat and Garen....I'd be okay with an extension to the Twist of Fate scene between them as another short, but otherwise? Eh.
Shorts involving random moments or events in a champion's life would be interesting, and probably the best route to go. Example: Orianna and Amumu both deal with being somewhat different, ostracized, and just generally not fitting in with the rest of the League's champions.....have they ever met each other?
Rammus doesn't say much. But that doesn't mean he doesn't have thoughts on things. Let's hear what goes on in his head during a variety of situations.
Pantheon wants to be a baker. Apparently Morgana burns cookies. Clearly we have an opportunity for SOMETHING here.
(while this has been done by various NSFW sources....) Rengar's on the hunt....but so is Nidalee. Who's Top Cat of the Jungle?
I think shorts, addressing random topics like these, would be better and less confining than an ongoing series with a plot and expository elements that fans would expect Riot to adhere to at every point during their creative process.
