More SF Stories That Need Sequels

“Breeds There a Man…?” by Isaac Asimov

In the original, extraterrestrials (or whatever) are treating humanity as bacteria that must be contained and have surrounded human minds with “penicillin” that keeps us from thinking in the wrong directions. In particular, they're trying to keep us from devising defenses against nuclear weapons.

In the sequel, SDI was the result of “gain of function” on the part of the extraterrestrials. They eventually see that humanity is now penicillin resistant. They mix up a stronger batch of their older disinfectants: irrationality and authoritarianism.

“Judgment Day” by L. Sprague de Camp

In the original, Wade Ormont, a Mad (positively furious) Scientist plots to destroy the planet in revenge against the rest of the human race.

In the sequel, he went over his calculations the next day and found the error. While trying to distract himself from the failure, he overhears a conversation between AI researchers and realizes that it will be easier to replace the human race than destroy it. Now that he has something to live for, he switches fields to AI, with a few side forays into trying to make sure civilization can survive a nuclear war long enough for an AI takeover.