A new Plugins menu in the menu bar with entries for Get new Plugins, Open Plugins Folder, Reload Plugins and a list of all installed plugins would be useful. Every installed-plugin-menu-item should have a submenu with About this Plugin, Preferences, maybe by pressing the option key an entry edit this plugin and below that a list of menu items defined by the plugin (see interface builder mockup).
in the case of applescript new event handles like about plugin menu item, plugin preferences menu item and build plugin submenu would be needed, and similar ones for the other languages supported.
