One word: How?

The Galaxy games also had 2D sections.

Or you could take after the 3D World game style in Super Mario Maker 2, but have the play area open up from 2D into 3D in certain areas.

We had Mario Odyssey, which have 2d platforming sections. Maybe make something like sonic generations, with a modern mario for 3d sections and classic mario for 2d sections.

I mean some parts of the level 2d and some parts 3d like the only the switch is a warp pox or pipe