Some open questions:
- How do you reduce the detail of a toon-rendered 3D model as the camera zooms out? How do you seamlessly transition between its more-stylised and less-stylised appearance?
- Hand-drawn 2D animations often have watercolour backgrounds. Can we convincingly render 3D scenery as a watercolour painting? How can we smoothly animate things like brush-strokes and paper texture in screen space?
- How should a stylised 3D game portray smoke, flames, trees, grass, mud, rainfall, fur, water...?
- Hand-drawn 2D animations (and some recent 3D animations) can be physically incorrect: the artist may subtly reshape the "model" to make it look better from the current camera angle. In a game with a freely-moving camera, could we automate that?
- When dealing with a stylised 3D renderer, what would the ideal "mesh editor" and "scenery editor" programs look like? Do those assets need to have a physically-correct 3D surface and 3D armature, or could they be defined in a more vague, abstract way?
- Would it be possible to render retro pixel art from a simple 3D model? If so, could we use this to make a procedurally-generated 2D game?
- Could we use stylisation to make a 3D game world feel more physically correct? For example, when two meshes accidentally intersect, could we make that intersection less obvious to the viewer?
There are probably enough questions there to fill ten careers, but I suppose that's a good thing!