Untold Stories Behind Other Stories
Sometimes when you see a movie, read a book, etc., there's something that indicates there's another interesting story in this world waiting to be told. Think Obi-Wan's reminiscing about how Darth Vader "had been a pupil of mine before he turned to evil" and "was seduced by the Dark Side of the Force." Although the Star Wars prequels weren't all they could have been to say the least (I left Revenge of the Sith straight-up laughing after "NOOOOO!"), it's my understanding the Clone Wars cartoon (the various versions of them) is well-done and the fall of Anakin Skywalker could have been a great tragedy in the Greek sense. He has his tragic flaws...fear of loss, anger...and between the negative influence that drawing on the Force using these emotions has and the fact a dark lord has been grooming him since the age of ten, things don't end well for Anakin, Obi-Wan, Padme, the Jedi Order, etc.
Well, that can be applied to some other stories, ones much less well-known than Star Wars.
*I saw a high-school stage version of The Wedding Singer with a former girlfriend and there's a line where Julia reminisces that her young-Trump boyfriend Glenn used to be much sweeter before he got so much money. There might be a story there...Glenn could have entered the New York business world a much better person (perhaps emulating or working for a relative...there's a line in one of the songs "don't cry uncle unless your uncle's the CEO") and got seduced by the money, power, sex, perhaps drugs, etc. that this world offered and turned into a massive jerk. The Glenn we see in TWS is a selfish, controlling, narcissistic jerk, but the murderous warlord Darth Vader was once a pleasant little boy who loved his mother, had lots of friends, and wanted to free the slaves.
*Although I've never seen the Australian Western film The Man From Snowy River (at least in its entirety), I did become curious about it and found there was actually actually a live musical "arena spectacular." It tells broadly the same story as the original film--a penniless young cowboy proves his worth as a man and woos the daughter of a wealthy station owner--but look closely at the cast. One of the side characters is named "Harrison" and is a young cowboy who made a fortune betting on horse racing. The disapproving (until the sequel) father of the spirited young Jessica is named Harrison and he made the money he used to build his ranch betting on horses. The musical is actually a prequel to the film.
(Given how zealous he seems to be about keeping his daughter away from protagonist Jim Craig in the films, the love story from the "arena spectacular" might have ended badly and soured him on the whole "redneck cowboys wooing ranch heiresses" thing. Yes, Harrison is a controlling ass whose behavior alienated his brother, his dead wife, and his daughter, but if it turns out there's some actual wisdom hidden in the class prejudice, sexism, and narcissism it takes the events of both films to at least mitigate, it would make him more interesting.)
*In the Sharpe television series, there's a soldier named Harris who the other characters say should write a book. He's based on a real soldier, who wrote an enlisted man's account of the Napoleonic Wars entitled The Recollections of Rifleman Harris. How Harris "wrote" that book--an illiterate man, he dictated it to an officer friend who eventually got it published--could be an interesting story too.
(The fictional Harris is literate and dies at Waterloo in the TV series, but if Michael Crichton can do the "mostly dead" thing for Ian Malcolm in The Lost World--he dies in Jurassic Park but shows up in the sequel--so could Cornwell and friends if there's any interest.)
And there are plenty of other examples. The TVTropes "Hero of Another Story" and "Villain of Another Story" have plenty. The Avengers depicts Hawkeye and Black Widow bantering about something that went down in Budapest that I think could make a good basis for a prequel movie. In my Wastelands world (the first of many planned novels is under consideration by a major publisher, so fingers crossed), I have the villain Grendel's son Falki reference events I later turned into the novella "Son of Grendel. I also have another planned novella explaining just why Sergeant Ezekiel Thaxton fears the Blood Alchemy Host (an army of mutants bred in the shadow of an abandoned nuclear reactor) so much.