Soundwave on 28 April 2016
The first Splatoon was made just under 2 years, and this from the concept stage where they spent months trying out different ideas. So May 2015 - May 2017 is a fair amount of time and then they can follow Splatoon 1's lead and add more content throughout 2017.
If Nintendo is smart Splatoon 2 will come out very early in the NX lifecycle.
