"Dagger of the Mind" had a great character in Helen Noel. Assuming Marianna Hill was available, I would have liked to see her as a regular.
Just as Sulu was plausibly retconned from physicist to helmsman, Helen could have gone from psychiatrist to a regular medical doctor; psychiatrists do have medical degrees, so it doesn't even bend the rules of reality. The change would make her applicable to a wider variety of episodes.
I think she had great chemistry with Kirk, and there would still be room for McCoy to play off Spock. Think about it: Friends
was a half-hour sitcom and every episode had time for three two-character subplots. As a one-hour drama, Star Trek
could easily have accomodated a Big Four, counting Helen Noel in, instead of just the Big Three.
She had the leading lady charisma and talent that was less evident in Star Trek's
other females. Her involvement would have made the show more amenable to women as their audience representative, and to men as our love interest. It might have been great.