Please ignore my last comment here if you decide to go with a list of "disabled" and "enabled" plugins. With that approach, we do not need to provide a way to say that no plugins should be invoked at an extension point. With that approach, when an extension point has some default plugins, the plugins must be disabled in order to have no plugins. When the extension has no default plugins, nothing needs to be enabled or disabled in order to have no plugins. So, either an empty or nil slice works.