I do believe it's only a matter of time where we will see a Splatoon spinoff, since the franchise has proven that it is not only here to stay, but has established itself as one of Nintendo's premier IPs in just 2 installments.
As for what type of spinoff.
You know, I'd buy a turn based RPG that further explores Inkopolis and the world of the Inklings and Octolings. It could lead to some pretty unique, interesting, and cool characters from all kinds of races in the game with types of variety. Think of it like the different kinds of Toads, Goombas, Koopa Troopas, etc. from Paper Mario 1 and The Thousand Year Door. And I can easily envision all the different weapons and sub weapons being used in an turn based battle system.
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