Not sure if this falls under the 'custom brushes' frequently asked for feature, but...
It would be super sweet if you could apply [and maybe you can already] arbitrary 'textures' to an area. Like paint this area to look like denim, or fill this area with a 'rough looking' texture to its existing color.
[Example of this is the seashore prog for mac which does almost the described--I miss those ]
Thanks!
-R