I haven't seen any evidence yet but I absolutely believe there will be. It's Nintendo's biggest new IP in years and they're going to milk it. It's huge. Like huuuge for Nintendo. They already released multiple amiibos and they keep updating the game so it's clear they're keeping an eye on it.
We've barely heard a peep about the NX though. If that drops in the next 1 to 2 years I could see Splat 2 being a release title. If Nintendo surprise releases the NX earlier than a year from now I think that it would be too soon for the devs to polish a sequel game.
I don't they'd want to release a title that big at the end of the Wii U's lifespan either. The Wii U is already not a good seller (relative to other consoles, by Nintendo and others). So releasing it late when another console is coming would not help sales of the old or new console.