My guess is this will be similar to Splatoon 2 to Splatoon 1. A "new game" but one that is built upon the foundation of the previous game and even recycles some of the same stages.
A lot of people here were saying Splatoon 2 for 2017 would be impossible, it would have to be a port of Splatoon 1 because 2 years wasn't enough turn around time for a full sequel, but it was a new game.
My guess is they'll take the Smash 4 models and refresh/update all of them, tweak the gameplay, add a full blown single player story mode, and all-new stages/music with some Smash 4 stages returning. Add in new characters (Inklings from Splatoon, ARMS characters, Solid Snake, Simon Belmont?).







