Switch between 3D and 2D

[chrome totem]

Hi, I would like to create a game with a simple mechanic in appearance but it will be tough to implement I think.
I want to create a platformer/puzzle game where the first mechanic is to switch the dimension of the game. Like, for example, in 2D there is an insurmountable slope, then we can switch the dimension to 3D and then bypass the slope.
At first, the game will be like in the screens. The first one is in 2D with the slope and the second one is in 3D bypassing the slope.
Have you some tips to implement a prototype of this mechanic?

For more precision, I see the game like, in 3D there the Z axes (like in the third picture here) and when we switch in 2D, the Z axes is reflected into the Y axes and then the gravity change to fall on the 2D ground (like in the fourth screen)

[waxen topaz]

Sounds like paper mario. You can change the projection mode and fov of the camera at any time
you can chance gravity at any time BUT you will ideally stick to one physics system (3d and 2d do not mix)

If you want a pure 2d version of this then you would need to adjust how player movement works + gravity when this "swap" happens

[chrome totem]

But how can I do to change the view and the gameplay (from 2D gameplay to 3D, with Z axes) and the gravity from 3D and 2D? like, how can I do a "3D gravity and a 2D gravity"?

[waxen topaz]

well if you only use 2d physics and want what you showed in the first 2 images. You can have 2d gravity and move on x.
When "swapped" you can disable gravity for the player and then let them move on x and y

It may make more sense to say we change from a side scroller type input to a top down type input

[chrome totem]

oh yeah I see, it's not too hard like that, I think I will do that in the first place and then change the gameplay if it's "too simple"