It would be nice to implement history brush tool similar to the history
brush tool in Photoshop.

History brush does not paint with plain colour, but with pixels taken from
the same position in some historic (or future) state of the image.
Otherwise it behaves like a normal brush. By default it paints with pixels
of the image  as it was opened (or newly created), but any other state can
be selected in a history palette (Undo history palette of the Gimp). (The
palette should have a checkboxes to select a history level for history
brush use and it shall keep future states -- when you revert for example 10
levels back, last 10 levels should be for exapmple grey to make clear they
are in future, but selectable for history brush purposes).

This tool is very useful when retouching and can save a lot of time and
nerves. For example you apply a filter to the entire image, but then you
want to revert some small parts of the image. The history brush is the
quickest and easiest way to do it. Layers etc. can do the same work, but
harder and you have to plan it before , for example, apllying the filter.

If interested I can supply some Photoshop screenshots and further explanation.