It's now possible to copy (copy mask) and paste (paste into mask).
It would be great if, in addition, masks also be linked or instanced so that changing one mask would change the other instances.
Similarly, it would be great to be able to do this with any paint or fill layer. For example, making an instance of a paint layer affecting the color and plugging this into the bump with a desaturate filter above it. This would reflect the way programs like Maya work where a bitmap image can be connected into any channel (bump, color, etc.)
Ultimately in order for a user to manage these connections it would be nice to have a nodal graph view similar to Nuke or Mari or Maya, rather than the layer stack system of Photoshop.