Google Earth PhotoOverlays can potentially contain a huge amount of detail. I would like to see some method of applying annotations to points or areas of the PhotoOverlay, along the lines of the 'Add Note' facility at flickr.
On a broader theme, if the 'surface' of a PhotoOverlay could be treated in the same way as the surface of the GE globe, you could apply any of the GE annotation methods such as Placemarks, Paths and Polygons - maybe even other PhotoOverlays. Actually, this could apply equally well to ScreenOverlay and GroundOverlay elements, as well as surfaces of 3D models - in fact anything to which a coordinate system can be applied.
Imagine the possibilities. Worlds within Worlds!