What if Riot would just hire a mangaka for a SG-Spinoff series with weekly/monthly online releases?
Well obviously you can't really take a western comic artist for that if you want to stay true to the genre, although I am curious how the "magical girl" thing would work in western (specifically american, but possible even franco-belgian) comics. I'm not saying that Riot's artists couldn't do that as well (the promo-arts last time were amazing after all!), but they are needed for the game, and this would be more something extra, so "outsourcing" that might be the best idea for a spin-off.
For all those who don't know, manga in japan is serialized in magazines, mostly weekly or monthly (some also only 2-monthly), with 18-24 pages per week or 40+ pages per month. Usually these releases are black/white, color pages only being given for promo-purposes.
Then at least I had something in my week that I could look forward to from Riot in terms of releases, even if it has nothing to do with the "real lore". The appeal would obviously be a "modern AU" for potentially all champions that can mirror and reference the "real lore" but doesn't really have to. It can stand in the tradition of "Sailor Moon" with Lux as the protagonist, but it doesn't really have to. Now they only need to come up with some interesting villians that go beyond "void monster", and that's a series I'd read the hell out of.
Also, once you got enough chapter together, you might as well sell them collected as physical copies. I'm sure Riot could make a fortune here if this was done well - and with their merch-store they already got selling infrastructure anyways, so that wouldn't be a problem either.