"It had to be squids" says Splatoon producer Hisashi Nogami.
Splatoon was one of the biggest surprises to come out of Nintendo at E3, as a brand new IP - and a shooter at that. Producer Hisashi Nogami spoke with Game Informer about how the strange hybrid squid children came to be the game's main characters, rather than the more obvious choice of simply using established Nintendo characters.
When we had a really good long think about, would one of [our established IP characters] fit it? Or should this new squid idea be the way to go? We realized squids were definitely the only thing we could go with.
"With us, first we're making the game, and then we think what kind of characters we would use with it, and obviously, one of those choices is established IP," stated Nogami, going on to explain that "We found that it really had to be new characters to would fit this game we were making. It would be kind of weird if Mario got splattered with ink and then exploded."
But what about Super Mario Sunshine, where Mario had a water cannon that worked somewhat similar to the "ink cannons" in Splatoon? "Yeah, but it's not like he blew up or anything," said Nogami with a laugh. "First we started with the ability to shoot ink but then we added the ability to swim really fast through it, and as we kept adding more abilities to what the player was able to do, there was this kind of moment where we all realized, 'Oh yeah, this would work really well with squids.'"
"Making them squids, it made a lot of the actions easy to explain and also made it really fun, and it was also this idea where all the actions we wanted to include in the game could be built around the idea of squids to get them all in there."
So there you have it, the reasons why Splatoon because its own IP, and not Super Mario Paintball (though we have to admit that sounds like a badass idea).
Source: Game Informer