Again, nothing legally actionable in terms of slander or libel come to mind, but I'm willing to be reminded.
CO's original premise, if you'll recall, was about actually prosecuting an individual for the crime of treason, though - not anything having to do with slander or libel suits. That's a rather wild stretch, in my opinion, especially in light of the fact that DJT was a private citizen during the campaign and did not hold or represent any government office. Even now, he has taken no oath and assumed no office.