Shelf layer for symbols
Inkscape needs a way to handle svg symbols.
Symbols are groups which are destined to be cloned and reused in a drawing. They are not supposed to be rendered, but their clones should be rendered. When working with an object destined to be reused in a drawing, it is nice to be able to keep it within easy reach in a special place out of the drawing. This could be done by having a separate "shelf" canvas, on which all objects would be 'symbol's instead of 'g'. This canvas would open along the normal canvas. Dragging and dropping from this canvas onto the normal canvas would "clone and paste", while dragging and dropping from the main canvas to the shelf would create a symbol from a group.
Given the ability to duplicate windows, this could be implemented just by having a shelf layer instead of a whole canvas, with the properties stated above. It would be nice though if that layer was not in the layer hierarchy: it does not make any sense to consider it to be above or beyond another layer.