20Spock Almost Left The Series
When Star Trek: The Original Series ended, the fans spent years petitioning for its return. The franchise would eventually come back in the form of the movies, but there was almost a sequel to the TV show in the form of the unmade Star Trek: Phase II.
Star Trek: Phase
II was going to focus on an older version of the Enterprise crew, with almost the entire original cast reprising their roles. We say almost because Leonard Nimoy declined to return.
The role of Spock was going to be replaced with a new character, who was a full-blooded Vulcan named Xon. Elements of Xon's character were later worked into Data from Star Trek: The Next Generation.