Both Windows and macOS allow the user to add entries to the right click menus of the application icon in the macOS Dock and Window task bar respectively. Other platforms may have similar concepts.
On macOS we already support this through the non-cross-platform QMenu::setAsDockMenu(), and on Windows we support it through the defunct QWinJumpList from QtWinExtras.
We should find a cross platform API for this. e.g. QGuiApplication::setMenu()