1. There will not be a lawsuit from Brennan, except possibly for political posturing. He has no grounds at all, and I suspect he knows it.
2. The reason why some people are allowed to keep their clearances after they leave the office that required them to have a clearance in the 1st place is so that if needed they could be called in to consult on decisions they made while they were still in office. Which could require them to review classified material that led to the decision(s) they made. That's about it folks, there's no other good reason for anyone to continue to hold a clearance once they leave the job that required it.
3. There is no way on God's green Earth anyone in the Trump administration is going to consult with John Brennan on ANY issue, so he does not need that clearance for any good reason.
4. Revoking that clearance does not in any way, shape, or form, limit Brennan's 1st Amendment Rights. Clearly, he has no restraints about stating his opinion either before or after the clearance has been revoked, as we see in today's Sunday talk shows, where he has appeared I believe 3 times so far. He has every right to say whatever he wants to say about Trump, and that security clearance has absolutely no bearing on that.
5. What we have here is politics, Trump playing to his base and Brennan doing the same thing. It's ugly, but ultimately inconsequential.
6. However, there probably needs to be some attention to the policies behind security clearances once a person leaves the office that required it. Maybe a 1 year temporary status that automatically evokes the clearance unless extended by the President. I read somewhere that there are thousands of clearances out there held by people who no longer should have them, and that probably should change. One wonders how much classified data has been compromised by people who were out of office. Were the means to access and the necessary accounts and passwords changed so that such persons could not access classified information? I'm thinking it's more of a problem than we know.