I understand why they chose to bring back Khan as opposed to another one-time character for Star Trek II, because his motives for revenge were very clear and easy to incorporate into a film, but if you were to go back in time and suggest another one-off villain from the original series be made the antagonist in a Star Trek film, who would it be?
For me, it would be Charlie X. My idea is somewhat similar to TWoK — eventually Charlie X's powers become strong enough that he kills the Thasians, escapes and goes after Kirk for letting them take him. I wouldn't want this film to be made now, but it would've probably been a better more than the insomnia cure that was "Star Trek: the Motion Picture" or "Star Trek IV: Save the Whales".
Instead of Charlie X, another idea that's quite similar would be that Kirk didn't really kill Gary Mitchell in Where No Man Has Gone Before, he just wounded and disfigured him, which makes him seek out Kirk and Spock for revenge.
For me, it would be Charlie X. My idea is somewhat similar to TWoK — eventually Charlie X's powers become strong enough that he kills the Thasians, escapes and goes after Kirk for letting them take him. I wouldn't want this film to be made now, but it would've probably been a better more than the insomnia cure that was "Star Trek: the Motion Picture" or "Star Trek IV: Save the Whales".
Instead of Charlie X, another idea that's quite similar would be that Kirk didn't really kill Gary Mitchell in Where No Man Has Gone Before, he just wounded and disfigured him, which makes him seek out Kirk and Spock for revenge.