According to the 1968 book, The Making of Star Trek, Gene Roddenberry had already started making plans for a fourth season of Star Trek: The Original Series before it was canceled at the end of season three. And he had some serious dramatic ideas in mind for the characters, with one being bringing Dr. Leonard McCoy’s daughter, Joanna, on board the Enterprise as a regular character. And being that she would have been beautiful, Kirk would have been attracted to her, and the two would have possibly become romantically involved with her.
Roddenberry wanted to give Bones the chance to grow more as a main character and test the limits of his friendship with Captain Kirk. Being that Bones and his daughter were estranged, the doctor would have had to work on his relationship with her while dealing with the potential relationship his commanding officer would have with her as well. That would have made for some seriously dramatic episodes.
Captain Kirk didn’t have long-term relationships so imagine if he’d gotten involved with Joanna only for the relationship to end shortly thereafter. It would had to have affected his friendship with Dr. McCoy. Even if the relationship had gone on for several episodes, Bones wouldn’t have been too happy with it, considering he knew Kirk’s track record with women.
Bones would have wrestled with his desire to protect his daughter who possibly didn’t want to be protected, especially not from a father she barely knew. And all the while, he would still answer to Captain Kirk. This could have made for some really emotionally-charged episodes, and it sounds like Roddenberry had intended to take the show in a slightly different direction, possibly even having Kirk settle down a bit.
It’s unfortunate that we’ll never know how this would have played out. This would make a great plot for an ongoing Star Trek novel, though, so there might be a tiny ray of hope that we still get to know how this would have unfolded.