If I had to guess I'd say the extra style slot will go to Mario Bros 2 US/Doki Doki Panic.
It would be easier to implement compared to say a Galaxy/Odyssey style and would appeal of a lot of player's nostalgia.
Agreed. I don’t know how it could be anything but. That’s if there’s only one slot. If there are more than two then who knows what we’ll be seeing. How about the side scrolling levels from Link’s Awakening. Haha. Yeah, not happening, but they do use quite a few Mario elements.