Something that is more based on Splatoon and its characters rather than being a 1:1 setting. So character names and a very general overview are based on Splatoon, but then it goes in its own direction. And that direction is: alien invasion!
When thinking about what direction to take a humanization of the Splatoon universe, I thought back to the times of the humans in its setting... And thought, what if, instead of the ecological disasters killing everyone, it caused brutal damage and loss of life, but was able to be recovered from? And that is the world this gijinka series takes place in.
After the world was saved from the brink of destruction, it brought about a golden age of humanity. The world working together to save what could be saved and begin anew, healthier and more sustainably. A desire to prove that humanity was more than its shortcoming and could make the most of any situation... However, an old bit of data sent out to space in the olden days was found by aliens, who perceived it as a sort of threat, and came to attack the Earth.
There is a long war, but the humans eventually come out successful. However, it sets back basically all the progress humanity made in its golden age, as you can imagine the type of destruction and death an alien invasion + counter attack would cause. Cities are filled with youth gangs and crime, rural areas are slightly better off but have reverted to the lifestyle of large multigenerational & multi-family houses to make ends meet... and the outskirt areas are filled with rumours of nuclear mutants and scattered alien groups. Basically, it's a depressing and harsh situation all around, but people are trying to get back on their feet and not giving up hope for the future yet.
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Characters that are Inklings in the main series are humans. Ones that are Octolings are aliens. Ones that aren't one or the other, depend on what I feel like. The aliens are kinda "tentacle creatures," as their inspiration is octopi, but they're also shapeshifters who can take the look of humanoids... with varying success based on their skill levels. The main difficulty is the so-called hair, which usually just ends up being tentacles. Think, yknow, how Octolings look like, but with human proportions.
The aliens are hivemind-y, violent, and have no art or music of their own. They're creatures of order though, so are very hard for humans to understand and get to understand. Pretty much like the worst of how Octarians are pictured within the normal universe. But some of these aliens end up becoming fascinated with art, particularly music, which is why so many music artist humans get entangled in this alien plot. Aliens can also get just...bizarrely attached to certain people they come across. This habit is seen particularly often in hybrids, which yes, exist.
The Squidbeak Splatoon also exists, which is mainly a mini fed-group that is led by Craig Cuttlefish, a veteran of the war that likes to rant on about the good ole days to his young recruits. He believes that there are still alien settlements on Earth even though the government denies it, so convinces random youth gang kids to go and find evidence and prevent further issues from developing. This goes as well as you'd expect.