The thing is, the strenght of C2 is that you can make almost anything 2D wise, the event system is opened enough to let you program all sorts of mechanics, then the behaviors are making the mechanics lighter, and all of that with no syntaxic issue.
In 3D you would have to have some behaviors to help and due to the fact 3D maths is not as easy as 2D maths, the number of parameters will increase, and their definitions will be harder than your clasical 2D world parameters (angles+distance? You cannot define a movement with that alone, collisions? How would that work with a platform behavior, with a cube that does not spin? Even the language can become a barrier as those are not everyday's words sometimes), then the event sheet has to be as powerful too, while being understandable, not easy due to the targetted demographic of C2, also this would have to be used by people (I personnaly do not even care about doing 3D games, and most people are either targeting low end devices for html5 publishers, or doing small games so 3D is not an huge addition to be had at this moment in C2's targetted demographic.)
Or maybe just 3D graphics applyed to a 2D universe? I do not see that being really useful if it is just what it is doing really, but in that case a simple plugin that would display a 3d model instead of a 2D image would do the trick just fine, then you would use sprites for the collisions and logic of the game.