OK, after many deep and profound brain-things in my head, I decided to add three additional characters: micro (ยต), pi (ฯ), and bullet (โข). All of these are easy to type (on a Mac at least) and frequently needed in science, math, or when just printing a list.
I think that's it though. This seems like a good set, and will probably be final. (Speak now, or forever hold your peace!)