A reminder to Riot: you can end one story arc and create a new one from the dénouement. An example: Nami cooperates with Diana, an epic quest follows, during which character and world are built, then Nami gets her moonstone, returns it to her people, and then decides to come back to the surface to advocate against Zaun etc dumping all their toxic chemicals in the ocean. New plot in effect!
Of course, that plot is bad, but I came up with it in under a minute. You could start by taking a few champions you'd like to tell stories about, make a post on the forums where people could chat a bit about what story they'd like to see, give thought points, and tell what they'd like to have the champs do after the main story.
Eg.
RiotLoreDrone: So, we're currently planning to tell a story about Shyvana going to Zaun on a fetch quest by Jarvan IV. What kind of things would you like to see of Zaun? This is going to be a sort of outsider perspective on Zaun so that we can properly explore some of its quirks. How much of Shyv's internal goings-on would you guys like to see?
ForumPoster: Sounds cool! What format are you going to tell it in? I think a sort of Noir-esque comic would be nice, so we'd have visuals and Shyvana's personality would be reflected in its tone. I'm a huge Zaun fan, not so much Shyvana, so could we have her interact with at least one Zaun champion?
LoreFan: Yay, Shyvana! Could we get some interaction between Shyvana and Jarvan at the start or end? (Maybe both, so it'd bookend the story?!) Also, could we have something on Demacian court life next? I'd love to see how Shyvana deals with all the noble Houses (and how they deal with her :P ).
...and so forth. You could mine ideas (noir comic, something on Demacian nobility) from forum posts.