Conception
The Original Series was to be set around a five-year mission, something re-referenced at the end of this year's Star Trek Into Darkness. Phase II would have seen a second five-year mission take place, and it would have seen much the same changes as that which eventually went into Star Trek: The Motion Picture. By this I mean that the USS Enterprise would have been re-fit, and the crew would be invigorated by the addition of new crew members. However, this was partly not by design, but by necessity. You see, Leonard Nimoy refused to come back as Spock in a new television series. Two years earlier he'd released his autobiography, I Am Not Spock while the title was misleading (it referred to Nimoy and Spock as being separate entities and recounting his dislike for people being disappointed that he wasn't in fact his character), the merchandising for the character was an irritant for him and he simply did not want to return to television in that guise. So Roddenberry split the role in two. Instead of having a Vulcan first officer who was also the science officer, he instead intended on casting both a first officer and separately a science officer. In addition, there was to new a third new character and you'll probably remember this one it was Lieutenant Ilia, played by Persis Khambatta.