This idea would be similar to scale-based sizing, but instead would affect the transparency of a layer based on the scale (image below). In my case, I'm building a vector basemap that includes lots of local roads and building footprints. I would love to see them gradually fade out as I zoom out, and fade back in as I zoom in. Right now the visibility for these layers "snaps" on or off when zooming past an assigned scale. I think a fade in / fade out effect for various layers could be accomplished using a scale-based transparency.