The ability to have individual lines move up and down through z-order would make them very inefficient. (really, anything other than onbottom) Each one would need to be its own window, with a custom region. If you wanted them to be antialiased, that's even worse. You could be clever and combine lines with the same z-order, but it might be a little complicated to keep track of everything. (and anti-aliasing makes that worse as well)

I don't see how this could look very nice without fills at least though... Theoretically, you could have filled shapes bounded by lines and cubic splines, but I imagine the interface and point management code would be quite tedius to write.

I'd rather have some form of SVG implemented. At least then you'd end up with something semi-standardized after all that work.