Why not simply add an option in the Border menu bar at the side of “Advanced Settings” (it could be also in the Advanced Settings, but I would not do it) to uncheck “Uniform Border”? Doing so, a handle would appear at each path node at the intersection of the edges of the different border’s sides. By dragging these handles we could “adjust the border width at each path node” – as Christian Kramer (Plz make border feature smarter) tells us – and we could hereby even have borders with changing width along a single edge, I believe. But now, imagine that we could also add border-nodes (even if the shape/path would not have one at this point) or delete a node, or, generally, edit the border edge as we do with a path (making curves, etc.)? Likewise, the lines of limit between different border sections should be editable.
Simultaneously, at the inspector, the border would turn to a “compound border”, we could open a drop-down list of all the border’s segments, and individually edit the color, opacity and “basis/average” dimension of each segment. Optionally we should be able of selecting each section of the border to get active and visible at the inspector by clicking the interior of it, not at the edge, in the canvas.