One of the things I'm working on is going to require a very flexible curve/path system. One of the things it will need to do is to allow me to set up paths on arbitrary surfaces - to extrude a curvy loop out to make a pipe or something, and then to draw on that new curved surface and extrude again to make something sticking out of it, etc.

capn_midnight has fielded the most cunning idea of making all such curves purely in 3D space, and then projecting them onto the surfaces. This means I'll be able to create different kinds of projectors, and to reuse the same path with multiple projectors to create images on different surfaces. All quite cunning really. Kudos to him.