While plans were underway for Star Trek: II The Wrath of Khan, Gene Roddenberry had a rather strange script in mind
Producer Harve Bennett had constructed a plot for Star Trek: II The Wrath of Khan that everyone seemed to like…except Gene Roddenberry. In fact, Roddenberry essentially believed parts of the script would ruin Star Trek. He had his own idea for Star Trek II, and while unique, it would never have worked.
The premise was that the entire crew of the Enterprise would travel back in time to 1963 in an attempt to stop the assassination of John F. Kennedy. But there was a glaring problem with the idea. If Captain Kirk and his crew were to succeed, it would have changed the timeline and history as bad as Barry Allen did in The Flash when he went back in time. (He essentially screwed up everyone’s life, including his own.)
Basically, there couldn’t have been a resolution to the movie, and though if anyone could have stopped Kennedy’s assassination, it would have been the Enterprise crew, it simply couldn’t be allowed to happen as it would have reset history. And anyone who has watched The City of the Edge of Forever knows that history must be preserved. Even the slightest deviation from the timeline can have an impact that would change the world as we know it now, potentially for the worse.
And if Captain Kirk, Spock, Dr. McCoy, and the rest of the crew were unable to save President Kennedy, what would have been the purpose of the movie? Perhaps they could have learned something that would have helped them in the future, but an entire movie revolving around an assassination they couldn’t prevent would have been a hard sell to fans. Not to mention, it probably wouldn’t have been a film welcomed by the Kennedy family.