I'm considering doing it from the base that's already in there, basically extending the current suite of commands available to allow you to do stuff like custom lists (by defining a template then inserting it into some other control with custom data) and calling stuff on existing objects (resize for size/position editing, setColor on swatches/bitmaps, setBitmap for bitmaps, stuff like that) and doing these on a per-client basis during play while preserving the original GUI serverside.
I'd have to spend a while looking over the things I often do with GUIs to get a concrete featureset planned but I'd be mostly expanding from the code that's there given that it's a solid framework lacking mostly in interactivity - the easiest example I can think of would be a custom inventory GUI and if I could implement that with it I'd call it usable.