This is a great concept! Transitioning between 2D and 3D is great, and it has a lot of potential for you to expand on. I think that it might be easier for the player if the left and right keys moved the player in both dimensions. as if the player can only move in one direction at a time depending on dimension then having 2 different movement keys for each is kind of unnecessary. Lastly I think your style is great. You have something perfectly usable without any art assets, so good job on that.