I'm also quite surprised not to see a new mainline 3D mario game. Mario Odyssey came out not long after the Switch 1 release, it feels like the timing is right for another entry.
Apparently that "Director's Cut" includes additional cutscenes, a co-op mode and an English dub. Fortunately, it sounds like it's only a timed exclusive.
This would suggest that, like Nightreign, single-player may not be the focus, or it could not be available at all, though the latter would be surprising.