In my experience the "Bring to Front" and "Send to Back" options are often cumbersome and difficult to use. Selecting an object that is overlapping on top of another object when I wanted to select the object beneath it is often a source of minor frustrations. The ability to create sub-layers within a layer would be a handy feature. That way, it would be easy to control which objects were on top of other objects. The way I imagine it, the user could manipulate objects on all sub-layers within a layer by selecting the "main" layer (in the layer menu), but limit their selection and view to objects on an individual sub-layer by selecting that sub-layer.