Gene Roddenberry wanted to do a time travel JFK shooting movie for STII
Because of the flop of the first movie, Roddenberry was aced out of having any say in making of the second.
Roddenberry envisioned a story where the Klingons uses the "Guardian of Forever" to change earth History and let JFK survive the shooting, this changes earth history, leaving humans technology behind the Klingons, who now rule them.
But Hollywood took a bet on Bennett, who came up with this story line and saved Star Trek.