This seems like a fun idea and mix of 3D and 2D.
![Smiley](https://forums.tigsource.com/Smileys/derek/smiley.gif)
I really like the grainy shader, it gives the special look to the 3D elements, and the boss/host entering the scene is great.
If the player will often die in 3D while making mistakes in 2D, you should be careful to avoid repetition, which becomes annoying for less skilled players. Potentially, even subtle changes like the number of spikes protruding from the screen and their placement could improve the impression, not to mention varying the overall cause of death.
Is the 2D world connected with 3D world through some logic and mechanics? Like opening doors in 2D open them in 3D, in the same way that death in 2D world cause death in 3D?