Who could've played Number One if the role had been recast in 1965?
Interesting question. Not too familiar with a lot of actresses available for TV in the mid-60's, but the first name that comes to mind is Diana Muldaur. Even in her two gust appearances on TOS, I think she showed that she could have been the authoritative/imposing type of personality needed for the character of Number One. At least she certainly did later on as Dr. Pulaski on TNG who, IMO, was much more believable as a CMO than McFadden was as Dr. Crusher.