Splatoon Raiders was announced last June, and is expected to make headlines again soon. The spin-off, set on the Spirhalite Archipelago, could indeed be released this year.
This theory is supported by the game’s European rating. In fact, the Splatoon Raiders page on the eShop now displays a PEGI 7 rating, which tends to indicate that the launch is fairly close.
We might see the game again during an upcoming Nintendo Direct.
