What are some "unofficial" sequels/spin-offs that are worth at least knowing about?

For example, they don't exist in the same cinematic universe, so Silence of the Lambs is an unofficial sequel to Manhunter, because Manhunter was the first Hannibal Lecter movie. Or, there is a TV movie called The 10 Million Dollar Getaway which is about the Lufthansa heist, which was a major plot point in GoodFellas. In this movie, Mike Starr reprised his role from GoodFellas, somewhat connecting the two. Any other movies like that?

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