Ability to create Folder-like Groups within Appearance panel and apply Opacity and effects at this Group level.
Using the Photoshop model of layer folders, provide the ability to group fills and strokes within the Appearance panel. These groups could then have opacity, effects, etc. applied at the Group level.
E.g. a complex stroke pattern could be built within a group using multiple styles, and then that pattern could be "embossed" onto another underlaying style using Overlay or Hard Light. The overall effect could then be contained within a single path and could be saved as a single Graphic Style.