Love this. One idea that I haven't seen implemented in other drawing tools: the default behavior after drawing a shape is to revert to the select tool. This is rational but it can also pull you out of flow if you're trying to diagram quickly. It'd be useful to extend the keyboard shortcuts to the select tool, so that when I highlight a region with select, I can press, e.g. 5, and have a circle automatically filled inside the selected region, so that I can bail myself out and not have to draw the same selection twice.