One feature I'd very much like to see is the ability to tree-script the entire main application window. I envisage this as a third mode alongside "Novice" and "Expert", maybe called "Script". In this mode the presentation would be similar to the Settings dialog and driven by DUI/Treescript in the same way as the Automation section is at present.
I find that I am using Girder more as a Lua scripting/application generation platform and not using the traditional Actions tree very much. It would be nice to be able to leverage the DUI to write Lua applications with user interfaces.
Taking this idea further, on Vista we probably need the ability to store scripts (including treescripts) in user directories to avoid the need to edit protected files in Program Files\Promixis\. Also it would be nice to have a built-in script editor based on Scintilla like the current Scripting Action editor.