TOS had a habit of only using guest female crew if they wanted a love interest, with a few exceptions (mostly yeomen of the week and Masters, whose love interest role was removed as being too similar to the plot of Space Seed).And had this character received the Kelley treatment originally, her not being included in the main credits as an "actual" lead, the illusion of Roddenberry's casting being dictated from above could have been maintained, too.
Alternately, Kirk's contacts with alien civilizations might have called for a Contacts Officer of some sort, allowing the female de facto lead to be a guest star rather than a regular. Not every adventure would need involve inspired debates about cultural clashes... but the audience might come to look forward to those.
Timo Saloniemi
Both Carolyn Palamis and Ann Mulhall were in roles that could easily have been recurring. There are tons of episodes with weird alien life forms or strange civilisations. They could easily have appeared in four episodes per season each. Even Chapel could have stepped up as a trained exobiologist if they hadn't been so intent on making Spock such a Mary Sue.
Ann Mulhall could have filled a functional role with the right level of gumption. She was also the same rank as McCoy and, weirdly, also an engineer, apparently. That's a lot to work with.