First I have to say, I love the approach to creating icons of dragging components onto a workspace.
One thing I started coming up against though was the order-dependency of the things you drag on. You need to start in the correct order, since the same 5 things dragged on in different orders will give totally different results. So one idea would be to keep each thing in a separate layer, and allow reordering of layers, and combining layers (like photoshop does). Would need to save some sort of project file for the icon though.
This would be useful when you're working on a series of icons, say for owner-draw buttons, where you need the same icon but with subtle differences to indicate different states of the button.
Another thing that would be useful is a batch-mode method for creating multiple resolutions and colour depths. At the moment, if I change an icon at 48x48, I need to then delete all the other ones and recreate them. This starts to get a bit tedious (but not too much) when you are creating icons with lots of resolution and colour combinations.