I've got some polygons that I would like to essentially fade to nothing as you zoom in. This could be implemented as a visible range of some continuous sort or added similarly to the vary transparency by attribute setting. I could imagine some people might want other symbol attributes to change on zoom, too, but this idea is mainly for transparency.
The only workaround I can think of at the moment would be adding the layer multiple times, setting a different transparency for each, and changing the visible range for each. Oy!