From the moment Star Trek began, Spock was a character who captivated fans' imaginations. However, actor Leonard Nimoy was the member of the original cast most reluctant to return in future projects. So much so that before Paramount decided to make a feature film, Star Trek: Phase II was going to replace Spock with a new Vulcan character named Xon.
Source：https://www.cbr.com/star-trek-lost-vulcan-phase2-mister-xon-spock/
