EntireReadOnly: Allows to close the wiki for any write actions for guests, but logged in users can edit. Only Admins can create new users. + more (plugins and templates can count on this option and can use it to change the layout or behaviors)
(As a note, this idea was discarded once in the past, but I check the possibility to do this with a plugin and the consequences doing this not in the main code. I still think such a heavy impact on the behavior of the site should be included in the main code. The native code is pretty simple, wen don by plugin the code is a mess. All plugin or template coders should have a native option for a read-only functionality. If you see this different, give me a hint why, so that a understand your coder strategies better.)