I've submitted a comment/attachment for http://jira.jboss.com/jira/browse/GPD-143 that addresses where the palette root gets its icons but there's another few issues involved with getting a fully separate plugin to work on top of the designer. I believe this also relates to GPD-173.
I've done some recent work in this area and the issues I have not resolved in an elegant manner are these:
How should we determine the ordering of the palette groups? My solution was to hard code the order of palette addition in the paletteroot as: basic tools, tools from other plugins, advanced tools, transition tools. This was ugly. A category ranking could resolve that issue.
How should we determine property tab group order? When property categories are loaded from different plugins, ordering is problematic. It is much the same issue as #1. above.
Could a category ranking be the best way? This one I got around by including my category in the GPD plugin.xml - which I really prefer not to do.
We extended the editor to listen to override createById so that we could keep a registry of some of our sub-elements in the process.
That meant we had to change the editorClass for the paletteContribution, and override the editor with our own class name. This part wasn't too bad but still required a change to the GPD plugin.xml.
Before I enter an enhancement request does anyone have any feedback or is there a better way to go about this?