In the left-side pane, somewhere below “Tagged”, a “User-defined” main folder with the option to create various sub-folders, in which glyphs can be grouped.
At the moment, one has to juggle tags in order to access a particular group, or creatively apply colour. It would be nice to be able to keep together e.g. all glyphs derived from a particular letter, say Latin small letter d. It might also be a good idea to be able to sort glyphs in individual folders, without applying that sort to the entire font.
Not to mention that “d” was just an example, and such a search lists “double…” in the group, whereas a user-defined category would list only those glyphs that a user added there - sort of like Tagged, but without a visible tag, and added to a particular, user-named folder.