That sounds very interesting to me, especially the editor!
Would make creating complex guis much more easier, i suppose!
If you allow procedural pictures, you should also implement at least one other options for developers who are not familiar with procedural generation.

Also it would be interesting to know how much of a speed difference there is between panels and draw_-calls. in general, I would prefer panels, but if they are a lot lot slower then draw would be ok. Depends on if there is any widespread usage for which panels are too slow.