What stars from the mid 60s would you have liked to see guest star on TOS who never did? And what would you have lived to see them play? For the purposes of this thread, it should be someone who was at least fairly well-known in the mid 60s who conceivably would've been available for a TOS guest spot.
My nominees would be Robert Culp or Roddy McDowall. I could easily see Culp as a Starfleet Captain and McDowall as a villain or an exotic alien.
My nominees would be Robert Culp or Roddy McDowall. I could easily see Culp as a Starfleet Captain and McDowall as a villain or an exotic alien.