First and foremost, this concept is sick! I love it! I was particularly fond of the 3d swap mechanics and the graphics. the water looks awesome. In terms of visuals this is pretty good. Gameplaywise, I find that the game is fairly buggy- I put together a list:
-clipping through walls in 2d sections
-infi-jump with walls (I found myself doing this to cheese coins lol)
the springs don't always work
The camera also switches to some funky positions when getting coins in 3D, making it hard to see where you are going.
When I beat the game and clicked the exit button, the game crashed- but it was over at that point so it was fine.
This is more of a level design thing and not really a bug, but sometimes when I flip to 2D, some things straight up disappear (Like some coins and the bridge in early levels- I think there should at least be a sign of a bridge that's in the background, though I think it's still good intuition to swap into 3D. I got really passionate about this and made a visual below that kinda puts what I'm thinking into an image).
This gave me massive super paper mario vibes, with the charm from sonic 06. Honestly, once I found out the infinite jump thing, gameplay got a lot more enjoyable for me, as the jump height is pretty low. Again, I really liked the concept and was woah-ing at the 3d flip! With better control, I can see this being an excellent game. Wall jump/10