I have said it before & I'll say it again:
Screw "Mode", we need full fledged games built around elements of the lore & narrative. There's enough lore & lore potential to expand the world of Runeterra to other games out of League's shadow. I don't think League's format is very helpful for a story based campaign. And even if you make it work, I would still prefer games specifically made with the story in mind.
And regardless of whether it's just a mode or a game, they need multiple of them to cover various different parts of the world. A game about
should play differently than a game about
. A game that focuses on Demacia & it's champs won't have much of a chance to give focus to Shurima & it's champs. What I'm trying to say is that there are a lot of elements, storylines & characters in League's lore & it would be impossible to show them all love in one story driven game or mode without it feeling cramped. Not to mention the potential for biased development & them leaning towards creating more games/modes about specific champs while not giving enough love to other champs.
Riot is sitting on a gold of narrative potential to create a franchise of games, books, comics, videos, etc that could last us to next few decades if they play their cards right. I don't think a simple mode is enough for that.