I think actually Zelda + Splatoon is a better launch combo than Zelda + Mario. Zelda + Mario would cannibalize each other's sales because they are similar single-player centric adventure games.
I mean you could spread it out other ways too.
Maybe Zelda + Pikmin 4 for launch, Splatoon in March/April 2017, Mario in May/June 2017 works as well.
Again the first Splatoon, the game was largely made in like 16 months, really even less.
So it shouldn't take more than 2 years now for there to be a Splatoon 2.