Some ideas about what would be a fast and easy way for an artist to paint a 3D scene:
The main idea is to be able to paint directly on 3D objects.
The modeling structure of layered domes for distant background objects and simple models for objects closer in the scene could both be painted on directly. It would be important to be able to paint not only color and texture, but transparency as well. A clone tool would also be useful for repeating detailed textures like grass.
A nice set up would be two windows showing separate camera views.
One would be the main camera view of the scene that will be used in the movie, with a timeline allowing you to scrub through and stop at various frames. This allows the artist to see exactly what will be visible on screen at each point in time. The adjacent window would be a camera view of the scene or of individual objects that the artist could move through and tumble around. This would be the workspace for the artist to paint in. The idea would be that as the artist paints in the workspace, the changes show up in the other camera view as well, letting the artist paint details on an object up close, and see how it looks from the main camera, or whether it is visible at all. In this way, parts of the object that are never visible to the main camera don't have to be painted.
I like the idea of painting layered domes for background scenery such as distant mountains, trees, bushes, sky, etc. For objects more embedded in the action of a scene (such as a merry-go-round, car, or trash can) perhaps a simple model would work better. They could be as simple as boxes and cylinders or much more detailed shapes. Either way, they could be painted to appear to have more depth and shape than the models actually do. Even better would be if transparencies could be painted on these 3D objects. For example, starting with a cylinder for a merry-go-round and "painting away" or erasing parts of it to create the poles, which could then be painted with color to appear rounded.