Re: TNG episodes that would've been interesting to see on TOS
I wonder if Chain of Command might not be too cerebral for TOS. Seeing Kirk being tortured as brutally as Picard was, might make a more naive 1960's audience a bit squeamish. They might also wonder "why doesn't he just escape"?
This is all sort of an odd exercise anyway. Retrofitting an episode from a 80s-90s ensemble series of 9 players into a 60s show with 2-3 leads and lower billed supporting cast means major story revisions. Network censors and broadcast standards have to be considered in the context of the 1960s-- easily done, if you lived during it. The severity of Kirk torture scenes couldn't be as explicit as in Picard's. TOS did dabble with it a bit, using agony booths, neural neutralizers, agonizers, and various alien torture devices and experiments. "The Empath" was pretty brutal compared to what was being seen on other shows then.
Dr. Howard, Dr. Fine, Dr. Howard: For duty and humanity! --Men in Black (1934)