@Background Pony #DC7A
There already is a manga series, so the most possible anime would be an adaptation of that.
As a movie the most fitting plot would be a retelling of the first game’s story mode:
A newcomer arrives to Inkopolis, tries to make a name for themselves, and eventually gets recruited by Cap’n Cuttlefish to fight the Octarians and save the Great Zapfish.
The thing is, I can’t see aproper retelling of this without Cuttlefish rating about the “Octos” and the Octarians having anti-squid propaganda all over the place. Also, as far as I see the plot for Splatoon 2 and the fanart of it, Agent 3, the hero, gets hardened, and turns from an innocent, young Inkling to a bitter veteran.
In the first part of the movie, Agent 3 get defeated in their first Turf War and gets told by an Inkling from the other team “Go home, rookie. You just slow everyone down.”, and at the end defeates the same Inkling in Turf War, looks down to them and goes: “… pathetic.”