Soundwave on 12 August 2017
It's not a conventional sequel, it's more like a port of the original game with more content added in and some content taken out.
That said, Splatoon is my favorite new Nintendo IP of the last 20 years and it's going to be a monster hit because people need their Splatoon fix. Splatoon is not really a conventional, regular type of Nintendo franchise, it's really their first online-centric IP ever, and changing the formula too much would probably actually ruin it.
