It seems like adding extra 3D functionality to Anime Studio is a bit controversial, so please don't post if you are against the idea of more 3D features in Anime Studio. The way I see it is that any feature that can improve efficiency or quality of a finished 2D animation should be explored, regardless of whether it is a 2D concept or 3D concept.
I agree that Anime Studio's approach of spline connected points would translate really well to 3D. Here's an example of how I think it could work.Postality wrote: one more thing I'd like to add to my small list is:
3D Vector
Such as this program: http://www.erain.com/products/swift3d/
However with Anime Studio's interface it would be a lot easier to achieve this kinda thing than it would be in swift 3D...
Just to add a bit to this, the 3D vector part of Anime Studio would mean building something in 3D with the vector draw tools and being able to export it as the normal formats at render time such as AVI, and MOV.

Assume the above is a cube drawn in 3D space with the new 3D drawing tools. You could then select all of the points and add a surface, the same way you would add a shape in 2D. Similar to the 2D case, the curvature of the surface would depend on the curvature of the points it passes through. In fact, the maths for generating surfaces would be very similar to generating a line, just with a few extra variables. You would be able to control the outline and fill color of the surface using the same styles as 2D shapes. Since the program already has to model the surface, you could also add simple cel shading to the final render.

You could then add 2D shapes to the existing lines on the surface to add detail to the object.

You could make the object look more geometric by decreasing the curvature of the points.

I think this would be a much improved way of generating 2D style animation from 3D objects, as opposed to "cel-rendering" polygon objects. The objects would be much faster to create and you would have much more control over where the lines were drawn and what they looked like.
You could probably devise a way of controlling these 3d objects with the current bones system or a new 3D bone system. However, I think they would be the most useful for non-organic objects like vehicles, buildings, etc.