It would be very nice if there was a way to have multiple layers in am image 
that can share pixel data. This would be different from current duplicate 
layers in that changes one layer would alter the other. There would also be a 
way to promote one of these to a real layer. 
 
The use would be when the layer contains some image you want to appear in many 
places, like a cloned man, perhaps. It would be most useful with the 
implementation of transformations on the layers, as described in some other 
bugs, including my own bug #142987. 
 
The interface would need to be clear, and that might be difficult but its 
doable, for sure. They could be called Ghost Layers, perhaps. On creating one, 
a dialog would appear asking if you want to 'ghost' that pixel data, the mask, 
or both. The original layer would be exactly like any normal layer, but the 
newly created Ghost Layer would reference the original layer instead of store 
its own pixel data. A Ghost Layer's Attribute dialog might have a drop down 
list of layers, to allow change of what layer it copies. One might also be able 
to drag and drop another layer onto a Ghost layer to accomplish this.