In the world of Epileptic Trees, Fanon Welding is what happens when fans see Shared Universes and Stealth Sequels where there are none; a specific breed of fan theory which argues that a particular work somehow secretly shares a universe and/or continuity with another unrelated work, no matter how bizarre the possibility is and regardless of their respective authors' intentions. To support these theories, fans will seek out any supposed link between the works in question as proof that the events and elements appearing in one work were canon to the other all along.
In other words, it can be considered the act of engaging in unofficial Canon Welding.
This commonly happens with works that happen to come from the same author, as they often tend to share similarities in style; it is especially guaranteed if they also contain Company Cross References to one another or share some specific Recurring Element despite taking place in entirely different continuities. Another common source of Fanon Welding theories is typecasting, the practice of having an actor play the same character type in works they appear in. As such, expect fan theories about similar characters played by the same actor in multiple works all being one and the same, or somehow related, thus making said works connected to each other. (Though Playing Against Type won't stop fans from believing any character played by the same actor is one person, regardless). Other reasons may vary: maybe there is an expy of a particular character, or a character with a Reused Character Design, but the fans think the former is actually that very same character under a different identity, and believe the latter is a pre-established one all along. Or maybe the respective authors of two works happen to be friends, and both are fond of including Easter Eggs as references to each other's work.
When such theories come to be accepted by the fandom and incorporated into Fanon, expect to see many crossover fics involving the works claimed to be canon to each other. If Word of God confirms the works in question to be indeed related all along, then it's Canon Welding.
Compare with the Doppelgänger Crossover, a trope where multiple characters sharing common features, voices, and/or actors are either brought together or revealed to be the same universe somehow.
- "The Pixar Theory": Due to the habit of Pixar to include Company Cross References in their films (such as a carving of Sully in the hut of the witch in Brave), there is a widespread, longstanding theory that all Pixar movies are actually part of one and the same universe. This was eventually discussed and debunked by the producers of Toy Story 4.
- Disney Animated Canon
- Some people think that Frozen and Tangled take place in the same universe, as two background characters in Frozen were deliberately modelled after Eugene and Rapunzel from the latter movie. This also led fans to believe Rapunzel is also part of the same family as the royal sisters, usually as their cousin.
- There is another theory that Frozen and Tarzan take place in the same universe, and that Tarzan is the royal sisters' brother, who was born after the parents' boat crashed. This is debunked because we actually get to see Tarzan's parents who look nothing like the sisters', and Frozen II confirms that the ship didn't wreck in Africa.
- Nicolas Winding Refn's movies Drive and Only God Forgives are often thought to be connected (usually with the latter being a prequel to the former), because both movies star Ryan Gosling playing similar characters.
- Due to Event Horizon's story fitting, both plotwise and atmosphere, the tone of Warhammer 40,000, many fans considered the film as a prequel when humanity makes its first contact with the Warp and its dark powers.
- Fight Club: A bizarre but popular theory posits that the movie is secretly a Darker and Edgier sequel to Calvin and Hobbes of all things. Jack — the movie's protagonist — is theorized to be the disillusioned adult version of Calvin, and that Hobbes — Calvin's ambiguously imaginary friend — lives on in the form of Tyler Durden, Jack's villainous alter ego. Marla and Bob from the movie are also speculated to be the adult selves of Susie and Moe respectively, the latter presumably having developed his signature man-boobs as Laser-Guided Karma for bullying Calvin in the past.
- Get Out (2017): Fans theorized that the film takes place in the same world as Being John Malkovich due to both movies featuring people forcibly taking over other people's bodies as a plot point and Catherine Keener as an actress, also theorizing that Keener's characters are one and the same. The directors of both films liked the theory.
- The Rock: One of the main characters of the film is John Mason, an aging British MI-6 agent played by Sean Connery. Mason was meant to be an expy and a deconstruction of James Bond — sharing many traits such as being a Cultured Badass, The Casanova and a bonafide Escape Artist — along with being a deliberate Actor Allusion towards Sean Connery's fame as the first actor to play the James Bond character in film history. However, this also spawned a popular theory that claims John Mason is not a separate character, but James Bond himself, and "John Mason" is simply an alias he is using during the events of the plot.
- A popular theory by YouTuber Rhino Stew suggests that Snowpiercer is a sequel to Willy Wonka & the Chocolate Factory because of the numerous similarities found in the details of the characters and plots of both movies.
- There is a fan theory that suggests Life (2017) is a Stealth Prequel to Venom (2018), speculating that Calvin — the alien life form that the crew come across — is actually a Symbiote from the Marvel universe because of its similar design and ability to absorb its prey. This was partially fueled by the fact that the former movie recycles a minor shot from Spider-Man 3, which was Venom's debut in live action.
- A popular theory — proposed by science fiction writer John D. Clark in 1956 (making this trope Older Than Cable TV) and promoted by various other professional writers and scholars such as William Stuart Baring-Gould — suggested that Nero Wolfe is the son of Sherlock Holmes, generally the product of an affair with Irene Adler. As discussed on this Wikipedia page, the novelist John Lescroat created a character called Auguste Lupa, who is implied to be Wolfe himself pulling a This Is My Name on Foreign, and who is explicitly Holmes son.
- wildbow leaves references to his other works in each of his stories, usually in the form of one world having a warped retelling of the other world's story in a different medium. Since Worm and sequel Ward have a canonical multiverse, it's theorized that Twig and Pact note also take place there on worlds that haven't been accessed via portal yet, and the stories are a result of alternate dimension shenannigans.
- If a character from an entirely unrelated work just happens to possess certain traits such as a colorful, eccentric fashion sense, means of time travel, knowledge of past and future events, high intelligence and problem-solving, bizarre, reality-defying technology, British mannerisms, the ability to regenerate into an entirely new person/being played by several different actors, etc., you can be sure that people will start crafting crazy theories about that character secretly being a Time Lord/Lady of Doctor Who fame, whether jokingly or unironically. On This Very Wiki, it is an unavoidable meme to make these assumptions on Wild Mass Guessing sections to the point that we have an entire WMG page dedicated to them.
- The Big Bang Theory and Two and a Half Men have been thought to take place in the same universe, because in one episode of The Big Bang Theory the cast are watching Show Within a Show Oshikuru, an Anime series that Charlie in Two and A Half Men wrote a theme song for (and the same jingle is heard in the Big Bang episode).
- There is a longstanding (not entirely serious) theory among Blake's 7 fans that The Federation in Blake's 7 and the Federation in Star Trek are one and the same. Since it is canon in the former show that the Federation controls the media and deliberately distorts what is fed to the public to suit its agenda, e.g. Roj Blake's show trial, the theory claims that the adventures seen on Star Trek are also propaganda created by the Federation to keep the population sated. Supporting this theory is that the Federation symbol in Blake's 7 is essentially the Star Fleet symbol turned on its side.
- Theres a rather popular fan theory that Booth in Bones is the reward or shanshu of Angel. They like to point out how similar Booth and Angel are and its well known that the Buffyverse has countless different dimensions. Since the shanshu was translated as become human and live until he dies, these fans like to think Angel was given Booths life as his reward, only with no memory of his previous life. A few former Buffy alums having appeared in Bones just adds to it.
- Brooklyn Nine-Nine is often thought to be in the same universe as The Good Place, with Michael Schur's other shows sometimes being brought in as well. They share a ton of cast and crewmembers, as well as sharing similar values and senses of humor. The most common theory linking the two is the idea that Eleanor Shellstrop might be Jake Peralta's half-sister; he's confirmed to have at least three siblings he knew nothing about until he was an adult, and the two characters have very similar personality traits and quirks.
- There is a popular theory that The Prisoner is a continuation of Danger Man, because the two have similar protagonists, both played by Patrick McGoohan (who cancelled the former to star in the latter), and one episode of The Prisoner, "The Girl Who Was Death", is an unused Recycled Script from Danger Man.
- Despite the fact that they are Mutually Fictional in canon, fans of Psych and Leverage share the idea that Shawn and Eliot are cousins, mostly due to the fact that they have the same last name: Spencer.
- St. Elsewhere's twist All Just a Dream ending has resulted in a massive crossover theory called the "Tommy Westphall Universe" or the "Tommyverse" for short. Because some shows had crossovers with St. Elsewhere, those shows must be in the same universe and thus also a daydream; shows that crossed over with those shows, then, are also in the universe, and so on. This theory states that over 400 TV shows are within the same universe as St. Elsewhere.
- Officially, all the campaigns of The Adventure Zone are completely seperate from one another. However, fans like to link them up in fanon.
- One popular joke is that Balance exists as a fictional podcast within the world of Amnesty, which was given a Fandom Nod in the 2020 Halloween special, where Aubrey (from the latter) dresses up as Magnus (from the former).
- A lot of fans headcanon that Balance is linked with Graduation by having Gordy's adoptive parents be the liches Barry and Lup. Given that Gordy was abandoned for having necromantic powers, and was later adopted by loving parents who encouraged and nurtured his abilities, it makes sense.
- Some fans like to think that Among Us and the Henry Stickmin Series take place in the same universe (or multiverse, if the Omega Ending in Completing the Mission is anything to go by), mainly due to being made by the same company and multiple references in both games.
- It is not uncommon to see theories and fanfics that place the Bayonetta and Devil May Cry games in the same setting due to both being directed by Hideki Kamiya. Bayonetta even has a character named Enzo who shares his name with a character that appeared in a DMC novel and who was alluded to in the user manual of the first DMC game; the Bayonetta lore even states that the Bracelet of Time was created by a witch named Eva who made a contract with a "Legendary Dark Knight".
- BioWare fans like to believe that the developer's two major franchises, Dragon Age and Mass Effect, are connected, with Thedas simply being another planet in the Mass Effect universe. The fact that there are a lot of Easter Eggs and Shout-Outs in both series certainly helps.
- It is commonly believed that the player character of Friday Night Funkin' is the brother of Hatsune Miku, as both are blue-haired singers with an electronic voice. Further fueling the fans are the creators of Friday Night Funkin' jokingly calling it canon.
- Thanks to the Marvel vs. Capcom games, some fans have theorized that all or some Capcom games take place in the same universe. One of the games is even titled Marvel vs. Capcom 3: Fate of Two Worlds, implying that it is a crossover between two universes instead of several.
- Fans of Obsidian Entertainment have taken it as Fanon that The Outer Worlds shares the same universe as Fallout: New Vegas due to both being made by the same studio and taking place in a Retro Universe (The Outer Worlds setting being modelled after The Roaring '20s, and Fallout's taking place in one modelled after 50s-era America). Specifically, it is speculated that the former takes place in the future following the latter's 's Mr. House ending, due to the Advert-Overloaded Future of the setting pairing with Mr. House's hyper-capitalist leanings delightfully well.
- The YouTube channel Dimension Bros theorized in a video that almost all of the seemingly unrelated adventures in Poptropica take place in the same timeline. However, they made exceptions for the sponsored Islands and ones that are too meta.
- When the Game Grumps played Zombie Claus and noticed the house map in the game was identical to the one used as Madison's house in the H-Game House Party, they acted like it was the same house, remarking that it would be difficult to return to House Party and not be afraid of a zombie Santa Claus hiding somewhere. Fan theories in the video's comments included that Zombie Claus was punishing the House Party protagonist for being too naughty during the party, or it was just Frank in a costume chasing after you for drinking alcohol.
- The Walten Files: Since the series — inspired by Five Nights at Freddy's and Analog Horror videos by creators such as Squimpus McGrimpus — first emerged on Youtube, many viewers believed that the eponymous rabbit mascot of the in-universe Bon's Burgers restaurant chain is not just an expy of Bonnie from FNaF, but also is none other than Bonnie himself. More specifically, that Bonnie is actually a Canon Immigrant to the mascot cast of Freddy Fazbear's Pizza, and that Bon was his original incarnation in the past. This was later jossed by the series's creator in a community announcement; Fazbear Entertainment does exist in the continuity of The Walten Files, but has nothing to do with Bon's Burgers nor is it plagued by the tragedies and supernatural events of the canon FNaF continuity. As such, Bon and Bonnie are, and will be two entirely separate characters with no relation to one another.
- Comedy website Cracked likes to dip into this every now and then, jokingly attempting to find links between two works no matter how wildly unrelated they are; some of their theories include the claims that Snow White and the Seven Dwarfs is a sequel to The Lord of the Rings, that all movies starring Arnold Schwarzenegger take place in the Terminator universe, that Carl Winslow from Family Matters was present in Ghostbusters (1984) and Die Hard as a prison and security guard respectively (all played by Reginald VelJohnson, who tends to be typecast as a police officer), or that Beetlejuice is the ghost of Batman.
- Fans of Wham City works often like to place them in one big universe together, something the creators themselves have claimed is entirely possible but unintended. One recurring theory involves the character of Alantutorial, played by Alan Resnick. Alantutorial vanished at the end of the series, leaving it on an ambiguous note. Resnick's typecasting often leads to him playing characters very similar to Alantutorial, leaving fans to theorize that it's Alan post-series — most often, this is either the sculptor in This House Has People in It or the nameless protagonist of "Visitor Information".
- Cartoon Network:
- There is a longstanding niche fan theory that Greg of Over the Garden Wall grew up to become Greg Universe, as both are musically inclined brunettes that deeply care for their family, and much of Steven's personality coincidentally mirrors OTGW Greg's. The theory is immediately broken due to Halloween, which Over the Garden Wall takes place on, not existing in Steven Universe (confirmed by a joke about "some kind of candy-givin'-out holiday"), though most tend to ignore that since that can easily be overlooked.
- It's commonly believed that Over the Garden Wall and Clarence take place in the same town due to a few Easter Eggs between the two, the most obvious being a full shot of the town in OTGW being reused from Clarence with a few modifications.
- A large number of fans believe Samurai Jack takes place in the same universe as The Powerpuff Girls. (As well as Dexter's Laboratory to a lesser extent). Apart from the obvious fact that both shows had Genndy Tartakovsky involved in their production, much of this is due to how the ruins seen in the first episode of the former appear to resemble a post-apocalyptic Townsville — the main setting of the latter — as well as the identical resemblance between the eponymous Jack and Professor Utonium, leading many to think Utonium is the future descendant of Jack.
- Due to sharing the same creator, many believe that Danny Phantom and The Fairly OddParents share a universe. Building on this is the common fan theory (that was at one point going to be canon) that Danny and Timmy are related.
- Gravity Falls:
- It is assumed that the show and Rick and Morty are, if not in the same universe, then at least the same multiverse, due to the creators being friends and the countless amount of Gravity Falls references in Rick and Morty. Not to mention the inter-series Brick Joke shared by both shows.
- Gravity Falls is also commonly assumed to share a universe with Over the Garden Wall, due to similar aesthetics and dark, mature themes. A common theory is that Wirt and Greg are related to Dipper and Mabel somehow.
- Another series with which Gravity Falls is commonly portrayed as being in the same universe is The Owl House, partially due to the Production Posse involved with both shows, but also because of the Crossover Ship of Stan/Eda gaining popularity. Stan's known to have gotten married in Las Vegas to a woman who sounds very similar to Eda, leading many fans to speculate that it was Eda, albeit using a fake name. The creators haven't denied it when asked, meaning this might be the one Vegas Crossover with canon support.
- My Little Pony: Friendship Is Magic:
- Some fanfics attempt to merge previous iterations of the show with this generation. The first version, My Little Pony 'n Friends, is one of the more common ones, with the ponies usually hailing Megan as a legendary hero.
- Discord was voiced by the actor who plays Q from Star Trek and Discord and Q are both tricksters with extreme superpowers. Because of this, some viewers speculate that Discord is Q.
- The Magic School Bus: Because Mrs. Frizzle is sometimes implied to be Really 700 Years Old and her bus can change shape, some people have wondered if she's a Time Lord from Doctor Who. This is debunked in an episode with a "Fantastic Voyage" Plot, in which it's revealed that she has one heart (Time Lords have two).
- Primal is sometimes speculated to take place in the same universe as Samurai Jack, due to the shows (again) sharing a creator and the presence of dark magical elements in both shows. Specifically, it's a common suggestion that Aku is behind the rage-inducing potion in "Rage of the Ape-Men", the eponymous zombie plague in "Plague of Madness" and the witches' magic in "Coven of the Damned" (with the leader of the coven supposedly being a daughter of Aku). Spear, the caveman hero of Primal also shares some physical resemblance to Samurai Jack and is speculated to be his ancestor.
- Some people think that Rugrats and The Loud House take place in the same universe, because in the Loud House episode "Linc or Swim?", there's an old man in the background who's modelled on Grandpa Lou.
- Many people believe that Total Drama, 6teen and Stōked take place in the same universe due to the focus of all three shows being based around teens, each show sharing the same production company, much of the cast having worked on at least two of the shows (notably Jennifer Pertsch and Tom McGillis, who created all three shows), and how the three shows have been known to reference each other. Sometimes, Grojband is thrown in as well, due to being also produced by Fresh TV (but created by Todd Kauffman and Mark Thornton instead). Additionally, people like to lump the older Clone High (a series with zero production connections to any of Fresh TV's shows) with the aforementioned shows, as the art style of Total Drama was deliberately based off it.
- Due to Tuca & Bertie sharing the same artist and much of the same Production Posse from BoJack Horseman, as well as also starring Funny Animals, some fans speculated that they took place in the same universe. However, Lisa Hanawalt jossed this theory prior to the show's airdate, as Tuca takes place in a Denser and Wackier world than BoJack, which became clear when the show aired.
- Fans theorize that Ready Jet Go! takes place in the same universe as Molly of Denali, mostly because Trini (who has the same voice actress as Sydney in Season 2) believes aliens exist, and Jake bears a striking resemblance to Sean, meaning that Sean moved to Alaska and changed his name because he got tired of Mindy's teasing, or Jet cloned Sean and sent it to Alaska. Dinosaur Train is an In-Universe show in Jet if "Mindy's Weather Report" is anything to go by.
- BoJack Horseman: Since Reginald VelJohnson plays both Sergeant Al Powell in Die Hard and Carl Winslow on Family Matters, Todd believes they're the same person, with Carl Winslow being Powell's alias under witness protection following the events of the first two Die Hard films.
- The Simpsons: In the beginning of "Three Men and a Comic Book', while Marge drives Bart and Lisa to the comic book convention as they discuss comic books, the two siblings notice the resemblance between Richie Rich and Casper the Friendly Ghost, and come to the conclusion that Casper is the ghost of Richie after the latter committed suicide.