"Nintendo has not released a completely new main series"
...
"Brand-new
Super Mario experiences have been hard to come by in recent years, with the closest fans have gotten being the
Bowser's Fury expansion that was tacked on to the
Super Mario 3D World re-release."
But yea, of course Nintendo would be working on a new Super Mario game. A 2D game though? I don't know, but it would make sense for them to release something to go along with the movie, something four player would make sense, but I'm not sure if they're ging to be making more 2D, though it would be faster for them to crank out and release in time for the movie.
But it's Nintendo, so you never know, a new Super Mario Bros could either be:
3D Mario in the vein of Odyssey
3D Mario in the vein of 3D World
2D Mario
Or they could change it up once again and present something (kinda) new.