But what I'd really like : ) would be some way of storing the original source for a clone in a file. I use Illustrator / will use inkscrape almost exclusively to diagram the internal guts of digital systems, with lots of symbols for flip-flops and the like. Thus it would be great to have a system-wide "register.svg" file that I could use as a base for the clone objects. I'm somewhat familiar with SVG and XML, and realize that this might not be possible.
This is relatively easy to do from SVG perspective. The hard part is creating a widget for such a symbol library. The widget must show a scrollable resizeable table with previews (generated on-the-fly, with settable sizes) and names, and should support folders and drag-n-drop. When we have such a widget we will use it for markers, patterns, gradients, masks, and symbols. In each case the widget will show both those items defined in the current document and the items from a library that comes with Inkscape, and you will be able to move document items to the library or delete library items.
If you know where we can borrow something like that for GTK (maybe somewhere in GIMP, I haven't searched much yet) that might help. Of course if you can code it from scratch that would help too :)
Right now we might implement a symbol library using menus with previews, similar to the marker menus in Fill&Stroke, but those menus are rather clunky. They also cause unpleasant delays because all previews must be generated before the menu can be opened.