wombat123 on 27 December 2015
If I had to guess, I'd say that a Splatoon sequel would come no earlier than 2017 (realistically 2018) since Nintendo likes to spread out its A-List franchises out over a five year window at around two per year (probably 3-4 a year if the NX really is going to have a unified library) and 2016 and 2017 is possibly going to be covered with a 3D Mario, a Zelda Wii-U port, Animal Crossing, Pokemon and Mario Kart.
