Rotating 3D sprites is actually a huge amount of work. As soon as 3D objects can intersect, you need a depth buffer to render it correctly. Depth buffers don't work with transparency, which is a pretty critical point for a 2D engine. There are ways to solve that, but if you do that and also allow rotating the camera too, by the time you've finished solving all the follow-on problems you've essentially created a full-blown 3D engine. That's something we consciously want to avoid right now.