I think it would be nice if some GSOC student were to work on such a feature: Currently you can't draw outside a layer's boundary, and you have to tell GIMP to adjust the boundary yourself. I think an option can be added that, when set to "on", lets GIMP handle that in such a way: 1. any time a layer is moved, GIMP will after the operation set the layer boundary to include the entire canvas. 2. On save, GIMP will autocrop all layers.

what do you think?