It would be so nice to be able to have a slider for shape keys or blend shapes. Essentially this would allow us to texture things like eyelids or other facial expressions. Or even different forms of damage to clothes or props without creating whole new scenes. I know this would be hard with the current preserve painting options from the edit tab. A possible solution would be when you apply the shape key it creates an instance of the current maps and saves the layers you make on the shape key as a separate instance. Just wanted to throw out this idea.