We've had "TNG Episodes That Would Work on TOS" before, so why not this? Obviously, the entirety of DS9 wouldn't work on TOS because of the different settings, plot points, and story concepts. But some of the episodes definitely would. Let's discuss!
TOS didn't have a terrorist group made up of Federation citizens but a single episode dealing with rogue humans would be interesting (I don't know how Roddenberry would have felt about it, though). I can't imagine Kirk or Spock getting the wool pulled over their eyes by Thomas Riker (or the equivalent), but someone like Sulu or Chekov could easily play the Kira role. McCoy might be interesting, too, as he'd be a great foil for whomever the episode's villain would be.
- Inspired by Lt. Stiles comments about the Romulans having spies aboard Federation ships. I like the idea of a Romulan impersonating a Federation ambassador and sending the Enterprise
on a fake mission to start a war. He wouldn't be able to imitate other crew, but the story could be reworked to find another way to hide him while Kirk and company are searching for him. The story would work best if set shortly after "Balance of Terror," but the concept is believable and would work well for its era given the paranoia about Russia and Soviet espionage.
"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.
"The Ship" -
Would be similar to "The Galileo Seven" but could work well with either Romulans or Klingons as the adversary (Romulans might be a better fit). Instead of having a Changeling aboard the damaged ship, there could be a respected Klingon/Romulan officer in stasis who dies near the end when the unit fails. Spock and Scotty would work well in the Worf/O'Brien roles, with Spock suggesting that the wounded redshirt should be allowed to die to conserve resources and Scotty throwing a fit about Spock's lack of emotion.
"Children of Time" -
This could work with Spock in the Dax role, the lone living member of the Enterprise
crew after the ship crashes on a distant planet. Take your pick for who the injured crew member is. There'd be no Odo/Kira subplot, but a credible way of working around this could be found. I like the idea of young Spock secretly reprogramming the ship while giving everyone the impression that his older counterpart did it, perhaps because of misplaced guilt about what happened to his former shipmates. Vulcan or no, it would be hard for Spock to outlive his friends while knowing that his curiosity stranded them there.
"Empok Nor" -
Instead of a space station, have the crew visit an abandoned Constitution
-class ship that's been taken back to dock for repairs/decommissioning. Not sure how the mysterious assassins aboard the ship would be, but having a bunch of redshirts die could happen just as easily by having the ship randomly malfunction. Scotty again works well in the O'Brien role. Chekov would be a decent Nog.
"Sons and Daughters" -
A nice way to retcon both David Marcus and Demora Sulu into TOS before the films. Not sure how a 1960s audience would have felt about Kirk being an absentee father (not totally his fault as Carol hid her pregnancy from him), but this could have worked by having David decide he wants to be a Starfleet officer after learning about his father and Kirk rejecting him as a nerd with no street smarts or common sense.
Any other ideas?