"Homefront"/"Paradise Lost" - A nice follow-up to "The Adversary." Maybe Kirk and Spock are called back to Earth after Romulans are suspected of blowing up a peace conference between Federation delegates and some other power. Not sure who to use as Kirk's turncoat mentor, though.
Sounds like the perfect role for my "wish they'd done Star Trek" top choice, Leslie Nielsen, who was doing lots of bad guys in those days.
Anyone who only knows Leslie from his later comedies should see what he does with what is essentially a cameo in The Poseidon Adventure