Huge benefit of Splatoon: every game, the final boss ups its stakes and mass.
Splatoon 1: silly fun dance party to get back a giant power source.
Splatoon 2: dance party to rescue a kidnapped celebrity from brainwashing and reunite two best friends, and regain the power source.
Octo Expansion: a genocidal tyrant wants to destroy a whole city and must be stopped.
Splatoon 3: join together with a ton of fellow cephalopeople and the final boss from the first two games to stop a rocket from smashing into the planet Earth and wiping out the world's WHOLE POPULATION to start anew at the whims of the shady corporate executive who sent you to purge legions of the species of your small companion. Oh, and Captain Cuttlefish nearly dies.
Which influences my own Splatoon 4 idea...so here's my idea for the final boss.
The villain there is a corporate executive of a smothering Octarian megacorporation. She wishes to control the whole of the planet, starting by running the Octarians herself, desiring to kill DJ Octavio then take power of the Octarian army and start a merciless crusade against the whole rest of the world, enslaving every other non-Octarian species (and all slaves that outlive their usefulness meet their grisly end) and having them work in her megacorporation. And when the planet is nothing but Octarian dystopia, she plans to lead a crusade into space and spread an Octarian galactic empire where all that stand in her way will be killed and enslaved...and just to salt the wounds of the Splatoon leading the fight against her, she kidnaps Marie and plans to make the poor girl into her first victim.
Hearing of the villain's rise to insurmountable power and her plans to kill him, DJ Octavio rallies a good amount of his own army to fight her, and a good amount of Inklings and Waterdragons also come in to fight. This leads to a giant battle scene between those working under the villain, and the armies on the side of good. And to make extra sure to squash all opponents, the villain unleashes her secret weapon, a massive mechsuit loaded up with weapons that don't just shoot ink, but genuinely deadly weapons-grade material. As a result, the fight is made of two halves - the new agent character, accompanied by Off the Hook and Deep Cut, entering the villain's mechsuit body to destroy numerous power nodes and fighting endless Octoling workers that protect said power nodes...and, as I mentioned, Callie hopping into a mechsuit of her own and proceeding to fight the villain's mechsuit in a clash of titans for the ages.
Yes, I was listening to this song while typing this, could you tell?: