The Godfather isn't the type of movie to lend itself to fan theories. Everything is pretty well explained throughout the film, and nothing is left ambiguous. But even though it's an open-and-closed type story, fans have long theorized elements of its plot and characters.
Fan theories are a fun and innocent part of movie watching (even if they tend to ruin our childhoods), and making outlandish theories will never not be entertaining. Thankfully, Reddit is a gold mine for finding goofy, interesting, or otherwise solid fan theories that others would have never thought possible.
Tom Had Luca Killed
The death of Luca Brasi isn't ambiguous. While meeting with Bruno Tattaglia and Sollozzo, Luca is stabbed through the hand and garroted. His death is one of the most tragic and memorable of the entire series.
According to Reddit user u/ortegasb, it wasn't Sollozzo who killed Luca, but Tom. They believe that the scenes were told in a non-chronological order, stating, "I think the sequence was: kidnap Tom, gun down the Godfather, Interrogate Tom, Tom gives up Luca, Luca goes to the club and gets choked."
The Godfather Part III Is Michael's Hell
As this Reddit user notes, "The Godfather parts I and II are a rich, vibrant, searing, multi-generational epic filled to the brim with dark humanity, but for some reason part III is (by contrast) a goofy, embarrassing, melodramatic cringe-fest that bears no resemblance to them in any way, shape, or form."
According to them, this is because Michael Corleone died between Parts II and III and is now living in Hell, and the wacky change in tone is meant to torment the fragile ego that hungers for power and subservience.
The Godfather Is Possessed By The Devil
The Godfather films are inherently tragedies. As u/The_Telltale_Fart observes, the first Godfather is about the corrupting of Michael Corleone as he rises to replace his father, and Part II is about the debilitating personal effects being the Don has on an individual.
This theory posits that those who become Godfather are possessed by the Devil and that those who come to the Godfather for help are making dangerous pacts. The theory does contain some great points (like Kate calling their child 'unholy'), and it essentially becomes the core theme of The Godfather wrapped in a fan theory about demonic possession.
Sonny Survives
Sonny's death is probably one of the most re-watched scenes from the entire trilogy. It serves as a brutal cap to his character arc, and it proves one of the movie's most violent sequences. It's pretty obvious that he does not survive, but not according to u/Sicilianfamily.
As they state, "Vito Corleone is looking at Sonny's body double when identifying the body at the funeral parlor." Of course, the primary response claims, "This is the worst theory I’ve seen on the sub," so it's clear that no one should take this very seriously.
Michael Long Suspected Fredo
Fredo Corleone is arguably the most tragic character in the entire trilogy, and having his own brother killed is probably the worst thing that Michael ever did. Fredo's deception is revealed in Cuba, when he inadvertently acknowledges his association with Johnny Ola.
But as this fan theory posits, Michael knew about Fredo's betrayal long before Cuba. As they write, "The tone in Michael's voice when he introduces Fredo to Johnny Ola is one of suspicion," and "Michael's reaction to drunken Fredo admitting he knew Johnny wasn't one of a surprise shock, it was a confirmation he didn't want to be true."
Sollozzo Insulted Vito Corleone
The meeting between Sollozzo and Vito does not go well, and it leads to the attempt on Vito's life. As u/thecircularblue posits, the meeting was over once Sollozzo knowingly insulted Vito in front of his family.
They write, "When Sollozzo says, 'I need, Don Corleone, those politicians that you carry in your pocket, like so many nickels and dimes,' he's basically calling him a 'nickel and dimer' - someone of low level operation or of menial tasks." This insulted Vito (as evidenced by his cheek-rubbing) and ended both the meeting and any potential association between Sollozzo and the Corleones.
Michael's Effectiveness Came From Military Training
It's made abundantly clear throughout The Godfather that Michael has a military background, and according to Reddit user u/Whitepill-rescue, this background was invaluable in his rapid ascension to Don.
As they write in the headline, "Michael Corleone's effectiveness and superiority over other crime families came from his US military training which no other mafioso of his time had." They conclude, "That he was so successful is because he turned the Corleone family into practically a war machine. He was operating his family as a company of the US army in peacetime."
Michael Gave Sollozzo An Out
One of the famous scenes of the first Godfather sees an inexperienced Michael meeting Sollozzo and McCluskey at a restaurant and assassinating them both with a hidden pistol. But as u/Bince82 posits, Michael provided Sollozzo an out that he didn't take.
As they succinctly state in the TLDR, "Michael reintroduces the idea of the assassination and convinces everyone he can do it, then with a ton of seriousness and conviction gives Sollozzo an out if he can guarantee his father’s safety from now on. Sollozzo balks and seals his own fate."
Elf Is In The Same Universe
One of the movie's most bizarre fan theories, u/speedboat26 claims that The Godfather shares a cinematic universe with Elf. They believe that Sonny faked his death to leave the mafia, changed his name to Walter Hobbs, met Susan Wells, and had Buddy.
Walter's rough, take-no-nonsense persona comes from his time in the mafia and is a more peaceful remnant of Sonny's violent temper. Furthermore, Walter's son is named Michael, having obviously named him after his long-lost brother, Michael Corleone.
Good Burger Is In The Same Universe
According to Reddit user u/confusedmoon2002, The Godfather shares a cinematic universe with the 1997 comedy Good Burger, which itself was inspired by an All That sketch starring Kel Mitchell and Kenan Thompson. The theory is quite lengthy and wades some deep territory.
Thankfully, the user provides a handy TLDR at the bottom, which simply reads, " In 1955, Abe Vigoda's character, gangster Salvatore Tessio, escapes execution in New York and flees to California. He takes a mild-mannered job at a burger restaurant under the pseudonym 'Otis', and 42 years later, the events of Good Burger occur."