In order to improve discoverability of plugins that haven't been installed yet, we should list them in the 'Add New Items' dialog as disabled entries, similar to how entries that can't be added a second time to a panel are, and have an indicator icon which when hovered over would give a tootlip saying that it hasnt been installed yet.
While in theory this is a nice idea, it is in practice quite hard. 1) We don't know which plugins are available in which distro. (Not all of them are everywhere) 2) We cannot offer the user any help with installing plugins, distros have different package managers. My suggestion is that we update the related documentation page that users get to when clicking the "Help" button in the "Add item" dialog to refer to the list of plugins we have documented so far. https://docs.xfce.org/xfce/xfce4-panel/add-new-items Plugins are listed here: https://docs.xfce.org/xfce/xfce4-panel/start
We simply show them a notification icon on the entry with a mouse over tooltip that they would need to install plugin X, assuming the plugin name isnt changing from distros to distro. We dont have to deal with availability or package management, as that is the user's responsibility.
Showing the plugins without offering a way to install them sounds like a disappointing experience. I'll go with updating the docs instead.