So do you need help with character ideas, or setting ideas? Might I suggest looking to more sources for inspiration. Splatoon occupies the genre of "grind fiction", which also includes works such as Jet Set Radio, Bomb Rush Cyberfunk, The World Ends With You, High-Fi Rush, some Persona games, Zenless Zone Zero, and more. They share a similar aesthetic to Splatoon, and ideas taken from them would fit in and feel right at home in the Splatoon world.