Z depth / 3D space for points on path.
I do medical illustration for suture techniques. I use a path with a stroke to illustrate the suture (like a string), which need to overlap and show depth. I add a drop shadow to the path, but it simply goes around the entirety of the path's stroke shape, and doesn't apply above segments of the path which should appear below the higher sections.
There needs to be a z-axis system where I can assign a z-depth to each point on a path, which allows the drop shadow effect to apply to the same path on depths below it.