Dear Riot; when you finally do decide to make a movie, consider these things.
1: Don't make it live action.
Animate the entire thing. The recent music video shows just how damn good you are at it. No one knows the champs better than you guys, and so, no actor/actress will be able to recreate the same connection you can create from animation. This will also allow you to hire the same voice actors for the champs if possible.
2: Don't try to shove all of the champs into the movie.
League's roster is just too big for this to develop a cohesive storyline. Yes, you will disappoint some people, if not many people, of not showcasing their favorite champ. But, i think there is no other company that understands the meaning of "can't please everyone" better than you.
3: Use what you have.
What I mean is make the story canon. Don't try to write something completely unrelated to the lore you already have. Assassin's Creed tried doing this, and failed miserably.
4: Don't hold back on maturity.
If the movie isn't rated PG-13, or even R, you honestly did something wrong. League might have a teen rating, but all of the stories written thus far tell me that even you guys don't want to be that restrictive.
5: Use a story that's hardly been covered, but one that doesn't need any explanation.
This might be the most important aspect of this. I believe I speak for the entire playerbase when I say that we would love nothing more than a full blown movie adaptation of the Noxian Invasion of Ionia. Unfortunately, that's all it would be: nothing but fan service. If you want this to be a successful movie, you have to be wary of the people completely unfamiliar with Runterra.
This is where the Warcraft movie failed. They covered a very well known story within their lore, but if you were unfamiliar with WoW, you would have absolutely no idea what was going on.
We the player base knows why Noxus invaded Ionia (kind of). To us, it would make sense. But to anyone else sitting in that theater, they'd be totally lost.
This is why I HIGHLY suggest you choose a far more subtle story. I lnow you guys were thinking the same exact thing, and I'm here to tell you, that, unless you plan on releasing a series of movies, Noxus invading Ionia is the wrong way to go about this.
So, here are some suggestions:
Lucian's backstory The fall of the 3 Sisters and the Watchers The Ruination Nasus and Renek's Ascension The job that got Graves captured Burning Tides Sion's death The construction of Galio Lux's backstory The Weakest Heart (but more fleshed out) Jhin's backstory Zed and Shen's backstory How Vi became a Sherriff Urgot's backstory
I could list alot more, but my point is is that you guys shouldn't aim too big with this. Noxus invading Ionia, The Rune Wars, and Icathia are too big of stories to fit in only a single movie. Focusing on one region or even just one character would make the movie far easier to explain, and far easier to connect with, that way any audience can enjoy it.