It was "almost complete" all the way back in 2015. Knowing that it was a brand new Wii U title developed by Nintendo, it probably had a bunch of dual screen gimmicks that wouldn't work on the Switch and needed redesigning. Although, it still should've surfaced by now.
Yea, I think Pikmin 4 is a case of will happen, but it surely went back to the drawing board after being cancelled for Wii U. But I wouldn't count it to the games here, it never got properly announced.