Oftentimes in a movie there is some event or happening that is only mentioned or discussed, but not depicted, which would make for a good flick in its own right.
Star Wars already did this. A New Hope begins with reference to an adventure to steal the Death Star plans. It was only later that they made that into a movie of its own, Rogue One.
I’m sure there are others that either have been made, or would be interesting candidates.
Quentin Tarantino seems to love these. In Pulp Fiction, you could do a movie about the unseen boxing match, where a guy dies. Or the tragic POW who never gives up his hope of seeing his boy become a man. Or the series of choices that leads one to become a gimp.
And Reservoir Dogs never actually shows the jewelry heist, let alone the heroic cops who swoop in to save the day.
The story of the Men of the White Mountains (the ghosts that Aragorn recruits in the Lord of the Rings) would be good. Or any of so many things in the Lord of the Rings (how Gollum becomes Gollum, the whole opening scene in the first movie where Sauron is defeated, the Balrog under Moria…lots to choose from).
One where I thought it was good that they didn’t show it was in Captain America: The Winter Soldier. Falcon has just told Cap and Black Widow about his wingsuit, and how it’s stored in a Level 5 secure facility, or whatever, and do they think that’ll be a problem? Cap turns to Widow for a moment, and says “No problem”. And then they don’t show it, because, well, for Captain America and Black Widow, it really isn’t a problem.
There have been many spinoffs of the cryptic “Giant Rat of Sumatra” mentioned by Sherlock Holmes, and in fact I think this was even mentioned in a cartoon TV version of it. Would definitely be a good topic for its own film.
Marty McFly: “Doc, you don’t just walk into a store and-and buy plutonium! Did you rip that off?”
Doc Brown: “Of course! From a group of Libyan nationalists; they wanted me to build them a bomb, so I took their plutonium and, in turn, gave them a shoddy bomb casing full of used pinball machine parts…”
There was another thread about the “Tears in the rain” monologue in Bladerunner, and I pointed out I definitely want to see the movie that ends with “Attack ships on fire off the shoulder of Orion.” and " c-beams glitter in the dark near the Tannhäuser Gate"
That was covered in the Animatrix, actually - The Second Renaissance parts 1 and 2 is the backstory of the machine war.
It was, in fact, the humans who “scorched the sky” with a self-reproducing nanomachine swarm in a last-ditch attempt to deny the machines their energy source.
Was it Calvin or Bart Simpson that was involved in “The Spaghetti Incident”?
.
At the very beginning of Firesign Theater’s excellent Holmes parody by that name, Sherlock mentions that story, but that he “will never be tricked into telling a tale for which the world is not yet prepared… The public must never know!”. You can hear Watson in the background: “Well, I’ll just write that down…”
The story of how Hadley’s Hope got eventually over ran at the start of Aliens (I know a comic briefly showed this) My billionaire investor project would be getting the company that did Alien: Isolation to make a sequel called Aliens: Isolation where you play as Newt from the very first facehugger at Hadleys all the way to when the Marines arrived since IIRC they said it had been two weeks between distress call and the Marine arrival so there’s plenty of stuff to examine there.
Escape from New York, just the entire backstory of how exactly the United States finds itself in a losing war against the Soviet Union and the Chinese, and how Snake went from Medal of Honor winner in Leningrad to completely disillusioned soldier.
Red Dawn, the entire war that went on behind the protagonists, but especially the nuclear battle between the Soviets and Chinese that killed off 400 million Chinese.
Adventures of the Dread Pirate Roberts from The Princess Bride.
P.I. Eddie’s golden days with his brother from Who Framed Roger Rabbit.
The original quest for the cursed gold from Pirates of the Caribbean.
Mike Wazowski (Billy Crystal) and others from Monsters, Inc. doing a kid friendly stand-up feature.
Basically the whole backstory of how the other world in Coraline came to be.
That whole war against Sentinels that Wolverine was sent back in time to prevent in X-Men: Days of Future Past.
Hawkeye’s badass solo act for the years between Avengers: Infinity War and Avengers: Endgame. I understand they told this story via a miniseries taking place after Endgame.