I would like to see a way to rotate any brush (not just squares/rectangles 
and circles/elipses). For example, suppose you created a brush by 
greyscaling a flower, then in Script-Fu/Selections/To Brush you create the 
brush. Now, when you go to use the airbrush (or brush tool) it would be 
nice to be able to rotate the brush at any angle (suppose the flower had a 
big petal at the top but you wanted the big petal on the bottom, so you'd 
enter a value like 180 for 180 degrees or to flip it, or even 45 to put the 
flower at an angle). 

Another useful thing to add to this area would be a way to resize these 
types of brushes. Suppose you created a signature using a font and saved it 
as a brush as mentioned above. Having a way to enter a size percentage or 
pixel width/height would be nice. For example, maybe I'd want the signature 
to be only 50% of the original brush size, or maybe I'd want the signature 
to be only 100 pixels wide (the height would be auto-adjusted in this case) 
so that it could be just stamped into an image like a watermark.

These would be nice to apply to any brush including those that have a few 
brushes in one file, including multi-color brushes like the vine brush that 
comes with GIMP. This would eliminate the need to make many of the same 
brush in different sizes and rotations.