Current scratch pad in Brush Editor is good for test new settings of brushes but bad for editing brush icons.
I suggest that we should have brush icon library where we can store foreground and background icons from selected areas on canvas or external images and create new icons for new presets with icon resource of the library.

Note: I use this Mock to upload a image for the Forum
Forum post: https://forum.kde.org/viewtopic.php?f=288&t=136777