The way I figure, I think Nintendo should go for it and make Splatoon 2 a NX launch title.
It should include all the content from Splatoon 1 (which shouldn't take long to port) and they can add on 6-7 new stages, 2-3 new modes, add a new quest in the single player, and then they can continue to add free content to it as time goes on.
It's just too hot of a franchise especially in Japan and there will be plenty of people (hopefully) buying an NX who didn't have a Wii U, so you'll have all these people for whom this will be a new game essentially.
Splatoon 2 + Legend of Zelda should ensure a good launch for the NX in fall 2016. Add in a tertiary title like a Diddy Kong Racing game from Retro or a Pikmin 4. Maybe an Amiibo Hub game.
Then a new EAD Tokyo Mario platformer can come in comfortably post-launch to ensure there is no dreaded post-launch drought. Mario Maker 2.0 should be out early in the NX cycle too IMO.
