Radial menus are a common UI tool for switching between functions quickly. Could be activated by holding click/tap on the empty staff – hovering over a tool on the menu and releasing would select the tool. I often find myself sticking to the Select and Touch tools, so this could give users more incentive to use the other tools we have available (Paintbrush, Eraser, etc.), and maybe inspire the implementation of newer tools (Group, Tune, etc.)
I really like this idea!