I think one useful feature could be symbols and instances.

The idea is like this: You right-click a layer in the Layers palette and turn it
into a "symbol". The layer is then taken out of the image and put in a Symbols
palette, which lists all the symbols in the current file. Then an "instance" of
the symbol is placed in the image where the symbol was. An "instance" is a
special layer that draws its contents from its symbol. From then on, you can
drag and drop several instances of the same symbol into the image, and they all
look like exact duplicates of the symbol image, and they can be
moved/scaled/rotated/transformed/flipped/etc., but this only saves the
transformation matrix with that instance, without actually changing the pixel
data. The instances' pixel data is read from the symbol, so you can't actually
change the pixel data of a single instance without converting it into a normal
layer first. But (and this is the important part) if you change the symbol, all
its instances are automatically updated to reflect the change.