Obviously 2D Mario needs to be on Switch. It'd be insane for them to not capitalize on the massive Switch userbase with 2D Mario. Switch has plenty of life left in it so there is no reason the only 2D Mario on the system should be a last-gen port of a pretty dang mediocre game, that was released kinda as an afterthought after the holidays one year with little fanfare and still easily went over 10m. Even if it comes out only 12 months before the next system it'll probably sell 15-20m. And anyway presumably we'll be getting a 3D Mario at launch of the next system.
2D Mario would make a perfect holiday title this year, though it would also be really cool if they were keeping it under wraps until the last minute and soon announced its release to coincide with the Mario movie release. That would make a lot of sense too, though I feel like they would have announced it by now if it was coming along with the movie in April.
As long as it's not another entry in the 'New' series. Making 2D Mario perhaps the last mega-selling Switch game to release would be a fine way to start a new 2D Mario series.