I propose adding a "log out globally" button on user pop-up to fast log out from all Stack Exchange pages. Why enter a OpenID page and then logout if we can have one button ?
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This cannot work, because you need to log out of your OpenID provider. That is not a Stack Exchange function, nor can Stack Exchange do that for you.
For example, if you are using Google as your OpenID login to Stack Exchange sites, the link you propose would have to log you out of Google. That means you would be logged out of GMail, Google Calendar, Google+, etc., etc. That would be a heinous breach of site security.
Of course, there is also a Stack Exchange OpenID provider, which theoretically could support such a link. But that doesn't mean that users use only that provider; it is perfectly feasible to have both Google and Stack Exchange Open IDs added to your account. Which one would be logged out when you use that link?