I'm with Bob Justman on this: he balked at the idea of Kor returning. It's a big galaxy, not a small universe.
On the other hand, I think "The Trouble with Tribbles" and "Day of the Dove" would have been better episodes if they had guest starred Kor and allowed Kirk to develop an ongoing relationship.
And Star Trek VI: The Undiscovered Country would have been a better film if Chang had been Kor, no question.