Adventures of the Galaxy Rangers, because of the relatively small size and inter-connectivity of its fanfic writing base, has a lot of solid fanon elements:
Giving Niko a surname, Dal'Ariel (Niko Dal'Ariel meaning "Niko, student of Ariel"). When Koch Vision sent out a press release regarding putting the series on DVD, they included the fanon surname. The fanbase then had to admit to Koch that it was "just" fanon.
Among GR's fanon: Doc wasn't always on the "right" side of the law, is a lapsed Catholic, and is the black sheep of a very wealthy family. Said family is usually quite large, with Doc having a gaggle of sisters. Zach's father was a nasty drunk, and that he's closer to Eliza's family than he ever was his own. Niko's Doomed Hometown was destroyed because of anti-telepath prejudice. Also, fanon states that the Rangers are loved on frontier worlds, but barely tolerated on Earth.
"Gum" is a god/creator figure to Gummis, making expressions such as "By Gum" or "Oh Gum" cases of Oh, My Gods!.
The Adventures of Jimmy Neutron, Boy Genius: It's widely believed that Cindy's mother is abusive; generally the abuse is limited to verbal and emotional abuse but the woman isn't above slapping Cindy if her daughter decides tocall her out. Sometimes her dad is abusive or an awful parent but this isn't as consistent.
For a long time fans assumed that the Lich was the cause of the Mushroom War. Season five would go on to reveal that he was actually created by the war.
Most fanartists who draw Finn in a realistic or Animesque style ignore that he has bad teeth, he's a bit pudgy, and his hat is made from a bear's head; instead drawing him as a Bishōnen with normal features, perfect teeth and a fluffy fabric hat (or none at all). It's also interesting to note that fanart usually shows some of his hair sticking out from under his hat, which only occasionally happens on the show. (This was popular even back when his hair color was only known through Word of God.)
Doctor Princess is said to resemble Betty, leading to speculation that they might be the same person. This was Jossed by Word of God, and later the show itself.
Bisexual Marceline and Princess Bubblegum (or, alternatively, bisexual Marceline and lesbian Princess Bubblegum) was a very common fanon due to the high amount of Homoerotic Subtext between the two, even before Word of Dante confirmed that the two were a couple in the past.
Whenever you read fanfictions that center on Princess Bubblegum, you will probably hear the word "Candy Council" where she always discuss with them regarding the current issues on the Candy Kingdom, or how they utterly detested her relationship with Marceline for various reasons and wanted Bubblegum to break up with her ala Naruto's village council. Not only do we have no proof regarding if she even has/had a council in her kingdom, but she also stated that most of the Candy People are too idiotic to see the bigger picture, along with her own tendencies to work alone with only Peppermint Butler to accompany her.
The Adventure Time Wiki suggests that Shoko's ghost appearing as her mutated self is a manifestation of her regret over betraying Princess Bubblegum and stealing her amulet. This is supported by her ghost appearing normal after Finn returns the amulet to Bubblegum, as if her spirit is finally at peace.
Alvin and the Chipmunks: Fans are convinced that the unnamed Lady in Red chipmunk from The Chipmunks: Rockin' Through the Decades special (more specifically the "Crocodile Rock" and "Surfin' Safari" videos) is Charlene. Charlene was the original Chipette. She appeared in only one song, alongside Alvin, before being replaced with Brittany and the rest of The Chipettes the following year. The unnamed Chipette from the videos resembles Charlene (they both have blonde hair tied back into ponytails) but has never been confirmed as her. Still, you'll almost never find fanart of Charlene without her wearing the red dress of the girl from "Crocodile Rock."
Prior to the reveal, many fanartists depicted Penny, the titular character's love interest, who in the first half of the show was depicted as an unidentified creature in a "peanut shell" with antlers and black holes over where her eyes and mouth would be, as a doe with a removable peanut shell. This was Jossed with the premiere of "the Shell", which revealed her true form.
Humanization fanart of Darwin often portrays him as being black or at least brown-skinned while the rest of his family is white, since that matches their voice actors. This became Ascended Fanon in a couple of occasions where humanized versions of the characters shown in the show.
Certain fan images and at least one fanfic depict a future child of Gumball and Penny (a relationship between a blue cat and a glowing yellow shape-shifting fairy like creature formerly hidden in her "peanut shell") as a cat with green luminescent fur (a mixture of his parents colors) and occasionally wings like his mother, who also like his mother, spent the first potion of his life with his true form hidden by a "peanut shell", but with some cat like features still showing through.
Many Animaniacs fans think that Yakko is left-handed and lactose intolerant (canonically he's at most ambidextrous, and there's plenty of times he ingests dairy with no ill effects). Less accepted is that Wakko is hypoglycemic. The evidence is that he said it, but they've also said other things that aren't true, including things like the Warners literally being Dr. Seuss knockoffs, when they're just actors. The "just actors" thing seems to slip some fans' minds. All three of these pieces of fanon came from one observation of each, two of them throwaway gags (the two endocrine disorders).
The younger side of the fandom, mostly on Twitter and Tumblr, likes to headcanon all three of the Warner kids as LGBT: usually Yakko as bi, Wakko as non-binary, and Dot as a trans girl. Wakko being non-binary in particular is backed up by a gag in the comic◊ where Wakko claims to be of the "other" gender so that the siblings could all be allowed to board Noah's Ark (with only one animal of each gender being allowed on board), and some of the crew members like the idea of non-binary Wakko.
Some on Twitter have also headcanoned that Nora Rita Norita is married to Hello Nurse from the original series. Cora’s lack of a father and the fact that Hello Nurse was never shown to be in a relationship in the original series add to this.
Did Mr. Plotz pass away, or did he retire? Despite "Wakkiver Twist" showing his return, it is a non-canon parody.
Another popular one is Pinky & the Brain being a gay couple, seeing how much Ho Yay they have in this series compared to the original one.
A series of Atomic Betty books revealed Betty's surname is Barrett, though her parents were never given first names in the show or the books. Popular fan belief is that their names are Tanya and Quincy.
Legend has it that John Blackstar was originally going to be a black man, but that the network nixed the idea and he became Ambiguously Brown, instead (Filmation's Lou Scheimer later claimed he didn't recall such a thing, though, so it may not be true). At any event, the popular fan theory is that Blackstar is Native American, like Bravestarr.
He never outright states as much, but a majority of the show's fans take it for granted that Bob Belcher is bisexual. He's already Ambiguously Bi in canon, and his open-mindedness as well as numerous moments of Ho Yay within the series lead many people to do away with the "ambiguous" part entirely. Fans also take it for granted that Linda would not only accept this, but outright encourage Bob to act on it.
Though it's never definitively confirmed in the shownote "Human Flesh" has Bob deny it while Gene and Louise believe it, Tina is frequently believed to have Autism because of her single-minded obsessions with horses and butts, lack of social skills, odd body language and Creepy Monotone.
Captain Planet and the Planeteers: It's a common interpretation that Captain Planet, like Gaia, sees the titular team as his children. It's never explicitly confirmed as it is with Gaianote In "Volcano's Wrath", she watches the team race off on their latest mission and comments "Children. They make a mother proud.", but his consistently protecting and counseling them and occasionally dropping his normal demeanor briefly when they get in trouble feeds the flames.
Ever since the first real fanfic Rhyme and Reason was released in 1996, Chip's last name has been Maplewood, and Dale's last name has been Oakmont. In the show itself, none of the Rangers has a canonical last name, even Gadget's supposed family name Hackwrench can only be deduced from her father's name Geegaw Hackwrench, but the name "Gadget Hackwrench" is never said nor written in any episode.
Of Mice and Mayhem was the first story to make Tammy a nurse. This stuck with the fans.
This is more common with the main Disney Chip and Dale than this incarnation of them, but the two are seen as siblings (usually twins). In canon their relationship is unknown other than that they're close friends.
Archie of Class of the Titans is generally regarded to be a poet, and knowledgeable about such. This seemingly stems from the fact that he mentions having read some Greek poetry in his first appearance.
A lot of fanfiction has the story set in Boulogne-Billancourt, an area near Paris, France, because that's where the factory that the show's Factory was based off of once was. The name of the show's city has never actually been said. In the Code Lyoko prose novels, the city is named "The City of The Iron Tower, France". Which can be assumed to be Paris... despite the strange terminology.
Another thing that's never really stated is that (most of) The Men in Black, who appear throughout the show, are from Project Carthage. It's probably pretty easy to assume, though. Franz Hopper working against PC... men in black suits show up to take him away...
The computer game that Mr. Delmas is sometimes seen playing is never named in the show, but has been dubbed "Penguin Recaptiatior 3000" by fans.
It's been stated in-universe that, after one is devirtualised, an extended period of time must pass before that person can be revirtualised. It's never explained why, but many fans suggest that going to and from Lyoko is an exhausting process, and being virtualised more than once in a short space of time could be dangerous. Another suggestion is that the Supercomputer has to recompile the Avatars after devirtualisation. Word of God answered this question in a french interview: the Lyoko-Warriors can't revirtualize because they are too exhausted by the experience and need to rest before going again. Code Lyoko: Evolution quantified the cooldown period as twelve hours.
Even though the heroes were initially never named in-universe, the fans have came up with two names for the team: Team Lyoko and Lyoko-Warriors. While the first was never used in the whole series, Lyoko-Warriors became Ascended Fanon starting with season 3. Similarly, fans referred to William as a "Xana-Warrior" after he was turned into XANA's Dragon, though the term was never actually used in-story.
Although it is never directly stated, it is interpreted that hair of any color (such as Aelita and Anthea's pink and William's mother's green) is genetically possible in the Code Lyoko world, although colors other than black/brown/blonde/red are highly uncommon.
Franz Hopper's one dramatic gesture with his coffee cup has led fans to believe that he has a great love of coffee.
It's surprisingly common (or maybe not so surprising) for fanfic authors to make Ulrich's dad psychologically and/or violently physically abusive (with one of the fanfics with a page of This Very Wiki portraying the man as so violent and manipulative that he literally destroys Ulrich's identityvia destroying the poor kid's mind) despite there being no real evidence in-universe (and at least a couple of pieces of evidence against it: Ulrich only seems worried about disappointing his dad and is eventually shown to be completely unafraid of telling him where to stick it).
Fans are quite likely to emphasize Numbuh 3's Stepford Smiler traits as she grows, even having her be depressed as a teen. According to the epilogue, she does canonically become more serious with age. This might also involve her going through a goth or emo phase.
Due to a writer on FanFiction.Net, most of the fandom will write Numbuh 362's favorite food as cheesecake.
Numbuh 3 can speak Japanese. While her parents are Immigrant Parents and Word of God says that one of the original ideas was to have the character only speak Japanese, it's unknown in the final show if Kuki and her sister were ever taught the language.
Many fans believe that Numbuh 100 is The Steve, due to the fact they have the same hair colors and both are leaders.
Most fans believe Paulina's last name is Sanchez, that Lancer's first name is William (after William Shakespeare), that Ghost Writer's name is really Devan (meaning poet), and that Danny's middle name is James.
Fans are divided as to whether Vlad has a ghost sense or not.
For some reason, most of the fandom agrees that Tucker organised a massive school-wide (and sometimes city-wide) betting pool on when Danny and Sam would get together
Ember has two fanon backstories (with some changes depending on the writer). The most common one is popularly accepted as official, however it's actually a fan creation: Ember was an unpopular kid who was asked out by a boy as a prank. He never came and she came home so late she was still sleeping when a fire broke out in her house. The second, even darker, backstory is that Ember was a teenage girl in The '70s or The '80s who slept with a boy who ignored her afterwards. At least two weeks afterwards, Amber killed herself by burning her house down. With both backstories, Ember's name is usually given as "Amber." Those that use the first backstory often add in that the boy burnt Ember's house down intentionally for a variety of reasons, rather than it being a freak fire accident.
Not a common one but a theory has been bandied about that Danny's female clone Danielle is not actually a gender-flipped clone but rather is genetically Danny's daughter; the explanation generally used is that shortly before creating her, Vlad obtained black hair that wasn't Danny (though there's no consensus on whether this was on purpose or not) and used said hair alongside a previously obtained DNA sample from Danny to create Danielle. This would explain why she's a girl to some degree.
At least one of the Mansons dyes their hair, which would neatly explain how the daughter of a blond and a redhead ended up with raven locks without certain possibilities coming into play.
In the 2010s, a popular theory (especially on Tumblr) began spreading around that Danny is a trans boy. While Danny dealing with having to keep his ghost half a secret has been viewed through Rainbow Lens, Danielle being a girl rather than a boy has helped to give this theory credence.
Dan Vs.: Dan and Elise have Only One Name. However, Dan and Chris are named for/based on the show's co-creators, and we know that Chris' last name is "Pearson" like his real-world counterpart. Fans will thus refer to Dan as "Dan Mandel," and for simplicity's sake assume that Elise took Chris' name.
DeMartino is almost always a military veteran, having usually served in The Vietnam War.
Several of the Background characters have been given fanon names and personalities, some of which were based on actual production notes or semi-canonical materials (for example, when we see Mr. O'Neill's seating chart in one episode).
It is often assumed that Tiffany is adopted, since she is Asian but has the surname "Blum-Deckler."
Jake's side of the family, as well as Tiffany, are often presumed to be Jewish due to their surnames. Word of God says he intended "Morgendorffer" to sound German rather than Jewish. For what it's worth, we see once that they own Christmas decorations.
Andrea's surname is either Hecuba, Thorne, or Hecuba-Thorne. Both come from a tie-in book where she draws a comic about a self-insert named "Queen Hecuba," which also mentions that she has a "beloved brother Damien" ("Damien Thorn" being the antagonist of The Omen). If Damien shows up in fanfic, he's usually younger than Andrea and also a Goth.
Apple White and Darling Charming are near universally assumed to be gay, despite no clear Word of God on the subject (and Darling's toy box text stating she does like boys). Some fans do see Darling as bisexual; however Apple is almost always seen as a closeted lesbian, never bisexual. Given the epic amount of Ho Yay they get with everyone around them, the kiss, and many other hints throughout the series, it's sometimes easy to forget it hasn't been explicitly confirmed.
Following on from this is the fanon that Apple has a unrequited crush on Raven Queen. It would explain a lot, and Raven deciding not to be Apple White's villain is treated like a break up, but there's nothing to suggest Apple's motives are anything other than stated.
It's become a common idea that Darling Charming is Apple White's Prince Charming, not her brother Daring Charming. Daring is instead the Beast from Beauty and the Beast. The latter theory has been confirmed. However Apple's and Darling's relationship hasn't been explored since they "kissed".
Face's Music Party: Despite this version of Face being explicitly stated to be the same Face as the one from the 90s, fans and detractors tend to treat the show's version of Face as a separate entity from the older Face, if not also as a younger sibling or relative to him.
Very unsurprisingly considering Fillmore is a preteen safe parody of cop shows, but fanworks that depict Ingrid and Fillmore as adults always have them as police or private investigators.
Anza's bodyguard training often means he gets sent on dangerous undercover missions and he and Tehama are frequently shipped together.
Futurama: In fanfics, Fry's middle name is almost always James or John. Officially, it's "Jay," same as Homer Simpson.
Gadget and the Gadgetinis, a sequel of Inspector Gadget, inspired a fan to post information about a false third series called "Go Go Gadgetinis" on imdb.com and Wikipedia. In this "show" Gadget is paired with a female Inspector Prince. In addition to providing an explanation for Penny's parents, they got in the crossfire between Gagdet and MAD and died, Dr. Claw is finally captured and Gadget and Prince are married at the end. The article on Wikipedia still references this hoax.
In Goof Troop canon, Pete is an extremely strict, emotionally abusive father to his son PJ, with a relationship based on fear and control, and PJ can't wait to move out of his dad's house. This is especially obvious when compared with Goofy's lighter, more open approach to fatherhood. That said, there is no evidence that Pete is also physically abusive. In fanon, on the other hand, it's generally accepted that he is as well, though the exact degree varies from person to person.
A common bit of fanon is that when Dipper's in his teenage/adult years he starts wearing flannel, much like the show's creator Alex Hirsch.
Many fans believed that Dipper would have lost his trust in Stan and Mabel for betraying him during the events of "Not What He Seems", the former from lying all summer and the latter from choosing to trust Stan over him. As a result, Dipper willingly submits himself to Bill's clutches. This was Jossed by the following episode, where its Mabel who expresses worry about their bond, while Dipper reassures her that nothing will ever ruin the trust he has in her. Likewise, he seems to have gotten over his worries about Stan once he learned his real motive.
When fans found out that Cecil Baldwin of Night Vale fame would be playing a character named "Tad Strange," they ran wild, speculating (based on Bill Cipher's AMA calling Tad "a real square") that the character was a square-shaped dream demon who was a rival and/or twin brother of Bill. Another Bill line, "I LIKE [GOMPERS] BETTER THIS WAY," led to theories that Tad was possessing him, and Tad was frequently drawn with a goatlike horizontal pupil in contrast to Bill's catlike vertical pupil; a bowler hat, a long tie and an umbrella were thrown in to complete the contrast. Some also assumed that Mabel's "floppy disc" shirt had been Tad's Arc Symbol all along. The revelation that the canonical Tad Strange wasn't actually important, instead just being a Ridiculously Average Guy with a Punny Name, resulted in fanon!Tad being reworked into a Recurring Fanon Character named "Dot Matrix," who occasionally retains some connection with canon!Tad.
While not accepted by 100% of the fandom, there is a good sum of fanon that has the Pine twins as identical twins with one being transgender. You're more likely to see Dipper as a trans boy than Mabel as a trans girl, however the latter isn't uncommon.
During the Whole Episode Flashback "A Tale of Two Stans," we see Stan and Ford's mom holding an unidentified baby. This is probably supposed to be the twins' grandfather, Shermie, but how did he go from a baby to having almost-thirteen-year-old grandkids in about forty years? A lot of fans thus headcanon Shermie as the oldest brother, and assume that the baby is actually Dipper and Mabel's father who Shermie was having his parents watch for some reason.
Some believe that Dipper wears hats to help keep his birthmark covered by his bangs.
It's pretty common for fanfic writers to portray Priscilla Northwest (Pacifica's mother) as an alcoholic, especially following the loss of the family fortune
Hazbin Hotel: With nearly 5 years between the Youtube pilot and the full series release on Amazon Prime, fans of Hazbin Hotel spent a lot of time filling in the gaps in character portrayals and potential development. This inevitably led to a lot of contradictions with the show when it was released.
Lucifer as a standoffish badass/actively cruel individual who mocks or demeans Charlie's efforts while his wife Lilith is the ultimately supportive and kind parent. Numerous fan songs take this angle with him and it was very much the direction people expected the show to take. Needless to say, Lucifer being a lovable goofballwho loves his daughter but was estranged due to his depression before becoming her biggest supporter while Lilith being a Missing Mom who first seen in series sunning herself on a beach in hell caught plenty of people off guard.
Anything about Irken mating is fanon. This includes various ideas, like that they lay eggs, males can become pregnant or, for that matter, that they mate at all, since from what one sees on the show it all seems to be done in a factory.
Dib and Gaz's last name is Membrane, since their father is "Professor Membrane." Word of God originally stated that Membrane is the Professor's first name, as they didn't want to give Dib a last name, but the comic continuation and 2019 film would canonize it as a surname.
Questions of Dib and Gaz's mom. There are two general theories on this: one is that they had a mom (usually looking a lot like Gaz) and that she died, often in some sort of lab accident. This can lead to angst for all the surviving family members, including Membrane (his workaholism is often linked to grief). The other theory is that Membrane created Dib as a lab experiment, a storyline Eric Trueheart claims they might have pursued if the show hadn't been cancelled. How Gaz fits into that theory is up to the fan to decide. A particularly common iteration is that Dib is a clone of Membrane, and often that Gaz is a clone of Membrane's deceased wife.
Many ZATR fans seem to assume that Zim and Tak knew each other before meeting on Earth; often this is taken so far as to make them former best friends. Zim makes no sign of even recognizing her when they meet on the show.
Word of God on the subject of Zim's age is that he is "older than any human alive." Somehow this extremely open-ended time range has mutated into "150" (which is rather near the lower bound), but "16 in Irken years", significantly narrowing the gap between Zim and his main "rival", relatively speaking. Needless to say, the term "Irken years" never appears anywhere near anything coming from the official show or staff, but that hasn't stopped zealous fans from listing it as fact on Wikipedia.
Fans just about unanimously refer to the species from Vort as "Vortians," though the show actually flip-flopped between that and "Vorts;" the comic eventually went with the former. It is also quite common to depict them as a species of scientists, which is never stated but rather plausible (Prisoner 777 and Lard Nar were both inventors and much of the Irkens' technology seems designed by Vortian slaves).
Stormer and Clash are near-always considered gay. In the IDW comics the former being lesbian is Ascended Fanon (and Clash is implied to like women), along with the Fan-Preferred Couple of Stormer/Kimber. Bisexual/bicurious Kimber, Jetta, Roxy, Pizzazz, and Minx also pop up as well.
Pizzazz, Minx, and Riot really get around. Minx is heavily implied to be this way but it's unknown if Riot's flirtatious attitude means he's the same way.
Rio is Latino. Word of God says it was unintentional for him to seem that way, however this became Ascended Fanon in the live-action adaptation and comic reboot.
If any character uses drugs in a fanfic, it's usually Riot due to his '80s rock star look, Roxy due to her background, or Eric due to stereotypes about CEOs. Alternatively, Roxy hates drugs and refuses to use them because her neglectful parents abused drugs.
Fanfic writers seem to think The Stingers all have casual threesomes, though canonically they're a platonic trio of friends.
Fanfic writers are in two camps on The Misfits: Either The Misfits are True Companions or there's a lot of Unresolved Sexual Tension and/or making out between them.
Jem and the Holograms will break up by the mid 1990s due to either internal conflict or the changing music climate. The Misfits usually hang around longer (at least until the early 2000s) before breaking up. Often Stormer will go solo afterwards, if she isn't the catalyst for the break-up.
In future fics Pizzazz, or more rarely Jetta or Roxy, will often be a single mother. Jerrica, Riot, Rapture, and Minx usually remain childless.
Stormer always has Good Parents, though at least her mother is usually written as deceased. Roxy almost always has a history of physical abuse, and sometimes sexual abuse as well; she was also raised without her father. Jetta tends to have a similar backstory to Roxy but to a lesser degree.
Fans ignore that Clash and several other characters naturally have blue, purple, and other unusual hair colors. Likewise people ignore Clash's purple hair for the red-and-blue she had in her first appearances.
Shana and Aja are orphans. In canon they only make one passing reference to their biological fathers (in past tense at that) and no reason is given for why they were foster kids.
Rio's trust issues stem from a poor home life, ranging from abusive parents to parents with relationship issues who argue(d) often.
One of the few tidbits we know about Stormer's past is that "love has been hard for [her]". This naturally leads fans to speculate she's had at least one bad relationship in the past. Those who pin her as gay or bi also tend to have her dealing with homophobia or gayngst growing up.
General consensus is that Miseryville is Hell or some other form of the afterlife/underworld. This was the original idea scrapped by executives for being too hard to sell to distributors, and is never stated in the show itself, but the fandom still views the the setting as such; the mysterious lack of normal humans other than Jimmy is justified as being that the citizens are transformed humans, while Jimmy himself isn't a monster due to his Incorruptible Pure Pureness and how Lucius cannot break him.
Heloise is the only other Miseryvillian who resembles a human, but her true nature is ambiguous. Fanon says she is a human who also mysteriously ended up in Miseryville (like Jimmy) or actually a monster that looks human, with half-serpentine demon being the popular choice as we never see her feet. Officially though, Word of God has only labeled her as "a bit of a shapeshifter".
A popular theory is that Beezy's mother and Lucius' possible former wife was an angel, explaining why Beezy isn't quite as evil as the rest of the Heinous family.
Because Justice League: Crisis on Two Earths is a Divorced Installment that originally started life as a Justice League tie-in movie that'd take place between season 2 and the Unlimited era with only a few changes (mostly swapping out John Stewart for Hal Jordan), fans tend to believe a Broad Strokes version of the events of CoTE still happened in the DCAU.
Similarly, there are a modest number of fans who consider Teen Titans cartoon to be set in the DCAU, despite it being officially not. This started out as a reasonable misunderstanding (Titans aired alongside League, didn't share any characters with it, was similar — if somewhat Denser and Wackier — in tone and art style, and had a mutual Shout-Out via swapping cameos from their versions of Speedy and Flash), but even nowadays after the confusion has long been cleared up, it isn't terribly uncommon to see people regard Titans as being part of DCAU... and, in fairness, it does slot pretty well into that continuity if you do some Broad Strokes kludging, filling in some unseemly gaps in the DCAU (namely giving a transition for Dick Grayson between splitting off from Batman and becoming Nightwing) and undoing the Exiled from Continuity status of the Titans and their associated characters (who never appeared in the DCAU aside from a few off-hand mentions).
There's a similar headcanon that considers The Batman a Prequel to Teen Titans. Both being animated shows not set in the DCAU with a similar animesque artstyle. It stems mainly from the fact that Robin in The Batman looks like a younger version of his Teen Titans incarnation (also because Batman makes a cameo in the comic book tie-in of the ''Teen Titans'' show, and looks a lot like Bruce Wayne in The Batman). In fact, it is because of Teen Titans that Robin wasn't adapted in The Batman up until Season 4. It's also worth noting that both shows have an episode that present a distant future where Dick Grayson becomes Nightwing, albeit with different designs, though it's arguable if either of these two futures are canonical. Both shows also have a Killer Moth, in The Batman he's an Ineffectual Sympathetic Villainthat mutates into a giant moth monster that's at least more visually similar to his Teen Titansincarnation.
Many fans of King of the Hill adamantly believe that Bill Dauterive is Bobby Hill's biological father—a popular theory that was never definitively confirmed or denied in the show. If true, the theory might explain Bill and Bobby's similar rotund physiques, and why Hank and Bobby have so little in common with each other. As supporting evidence: it's established that Hank has fertility issues due to his low sperm count and narrow urethra (which is why Bobby doesn't have any siblings), and Bill has been shown to have a crush on Peggy in several episodes.
In canon, Charlotte's parents are said to "be in space", with them being dead only being implied. Fanon chooses to remove any ambiguity and just has them as being deceased.
A common fanon theory is that "Vendetta" isn't her real name. Most will go with "Violet", after her mother Violeta (but never "Viktoria", which would work better since it's a common Bulgarian girl's name, and makes her named after her father Viktor.)
The M.A.S.K. fandom tends to pair Brad Turner and Vanessa Warfield together thanks to their clear chemistry in the episode "Cliff-Hanger". One fan even theorized that Vanessa was secretly working with M.A.S.K. for much of the racing series and was slowly turning against V.E.N.O.M.
A commonly accepted piece of fanon is that Offdensen is the voice of Facebones. They are voiced by the same person, Brendan Small, but so are a lot of other characters, so nothing can be proven or disproved. Although some fans have pointed to one moment in the episode "Murdering Outside The Box" where his voice directly comes out of his mouth, although how intentional this was is still heavily debated.
Another is that Offdensen was a fan of Snakes 'n' Barrels, Pickles's old band before joining Dethklok, and somehow (opinions vary on how) that connection is eventually how he became Dethklok's manager.
Many assume that Nathan is 1/4 Yanemengo through his mother Rose, as she looks more like someone who's half Amazonian than his very pale, blond father.
The Season 2 finale features a brief appearance from a tweenage boy that is introduced as "the label head's son" and in the next episode we're introduced to Roy and Damien Cornickelson. This has naturally led fans to speculate that this boy is also a Cornickelson (although him being Roy's younger son is a bit more popular than him being Damien's son) although the show itself never comments on it.
Fanfics often suggest that Milo and Melissa had a series of "third friends" before Zack came along, but that none of them could handle Murphy's Law and eventually left. Such fanfics usually have Milo living in fear that this will happen with Zack as well.
Fanart often depicts Milo as Covered with Scars; he's mentioned having scars in the show but they're not visible to the audience.
Bradley is often changed from merely being grumpy to depressed or even suicidal.
Technically the show never says that Bradley has a crush on Melissa, but their interactions in the first few episodes certainly leave that impression.
It's been popular to suggest that Dakota, who so far always wears sunglasses, has mismatched eyes that he's trying to hide.
Melissa being either a lesbian or bisexual has started gaining speed. Zack and Milo as LGBT is not quite as popular but also raising in popularity; Milo is frequently pansexual if he's LGBT, while Zack is either gay or bi.
Mort and Chad are often seen together and look so similar that most fans assumed that they're related. Canonically, they're not. Gets a bit of a Fandom Nod when Bradley and Melissa admit that they can't tell the two apart.
Early on, many fans were convinced that Orton was actually Melissa's father, apparently just on the basis that they have similar chins. This usually went hand-in-hand with the assumption that Orton would be some sort of Mad Scientist villain. As for Melissa's supposed dad, Richard, he was either Orton's boyfriend or a robot. All of this was Jossed when Orton actually appeared.
My Adventures with Superman: Due to a few hints throughout his introductory episode, many fans have stuck with the DC Rebirth explanation that all Mxyzptlk are the same one and thus this Mxy is just the original gauging the new Supes.
Wirt having some degree of demonic tainting from his adventures in the Unknown, though the degree of it varies wildly.
Fans seem to assume that Wirt would have slowly become some kind of "Beast" himself if he had taken the Beast's offer, even though the Woodsman had the Lantern for years with no such effect.
Wirt isn't close to his stepfather, which feeds into why he initially didn't like Greg. Whether he gets over it and repairs his relationship with said stepfather or if the man is a jerk who openly favors his biological son varies depending on the fan.
Wirt's biological father commonly is portrayed as a poor parent, though it varies if he's just neglectful or abusive (and if so, how). Canonically, all we really know about him is what we hear on the "For Sara" tape, which implies that he was at least involved in Wirt's life when he was younger. Interestingly, you'll almost never find a story implying that he's dead.
Due to Phantom Investigators having a short run and a small but dedicated fanbase, many popular theories have sprung up among fans:
Prunella being Daemona's real name is quite common among fans, as it came from an earlier point in production where Daemona's real name was "Prunella Daemon" but went as "Daemona Prune" as a PI. It's never specified in-series whether or not "Daemona" is just a nickname or not in the final product; many fans assume it is and go with "Prunella Prune" as her real name. Some fans even make "Daemona" her middle name, making her full name "Prunella Daemona Prune". It's also common for bringing up her real name to be a pretty big Berserk Button with her.
Word of God confirms that Casey doesn't have a mom, though what happened to her is unconfirmed. It's usually agreed among fans that she died sometime before the events of the series.
Daemona is usually the catalyst for the rest of the team becoming friends. And while how she met the rest of the gang tends to vary from fan to fan, generally it's assumed she met Kira first.
Navarro owns a cat.
Daemona's cousins are Loopy and Larry Cooper, via their dad being Daemona's mom's older brother. In a similar vein, Life With Loopy taking place in the same universe as the show, which is lightly hinted at in "Stall Of Doom" (Kira has a poster of Bugsteak, Larry's favorite band, in her locker), but otherwise wouldn't have too much confirmation as the two shows are owned by different companies.
Daemona is bisexual, due to Ship Tease between her and both Casey and Kira.
Daemona is generally believed to be the world's most Violently Protective Girlfriend for Casey. This is also the case if she's with Kira.
Fans who ship Kira with Jericho usually have her act as a mild Tsundere towards him.
Mary, Daemona's grandmother, died from some sort of disease. Navarro confirmed that she was aware that she was dying, however the cause was never stated. Meanwhile, her grandfather, Harold, is usually assumed to have been killed by Willard.
Navarro having known Daemona since she was a little kid tends to be a common idea as it's revealed he was part of the original Phantom Investigators with her grandparents. It's usually assumed he met her when they were both at Mary's house, visiting on the holidays.
Another candidate for Daemona's middle name is "Mary", after her grandmother. Kira's is usually given as "Nakisha", after her original name early in production. Jericho's and Casey's usually go by the first names of their respective voice actors, so "Aleksander" and "Andrew".
Not much is known about Daemona's life before she moved to San Francisco, though it's a common idea that she was bullied at her old school. Frequently Kira, Jericho and Casey are her only friends period though some fans may give her a single friend at her old school or have Loopy and Larry be her only friends (this can be paired with them being her cousins or be on its own).
Jericho having ADHD comes up now and again.
Daemona having magical potential, implied by her toy from the failed Carl Jr. kids' meal line, is a popular idea though exactly what type of magic varies (given the crystal ball, seer type abilities are frequent but not universal).
Daemona's grandparents, Mary and Harold, are often depicted as being a redhead and a brunette respectively, as their only on-screen appearance was in a flashback in black-and-white (one of the animators confirmed that the scene was shot in color, but photos have not surfaced).
PAW Patrol generally has two explanations for why Ryder is in charge of the titular team. A) His parents are deceased and are in some form responsible for the initial idea, which he expanded to honor their memory or B) one of his parents, usually but not always his dad, is in charge of a global/international organization with human and canine operatives with the PAW Patrol just being the Adventure Bay division. Ryder's mother has roughly 50/50 odds of being alive vs out of the picture in the latter scenario, with the same being true for his father if the mother is the one in charge.
Chase is particularly close to Ryder, possibly even having been his pet prior to the team's formation. Sometimes Marshall shares this closeness but not always.
Most of the team has been portrayed as an orphan by somebody though the consensus is that Ryder is legally responsible for them all regardless (or in the case of them being under his father/mother's authority, said parent is) and that they think of him as something of a father figure (or older brother if one of his parents is in charge of them).
Pounds are the dog equivalent of jails while animal shelters are basically orphanages.
Not over sweeping but Chase and Marshall at the very least have meaning to their respective numbers, marking them as Ryder's second-in-command and third-in-command respectively.
Dogs, despite the uplifting, have many similarities to canines in our world, including potentially lethal allergies to chocolate.
Wolves alternate between being normal animals (making the intelligence of dogs being tied to domestication) and just as intelligent and speech-capable as the pups.
Dogs age more like humans than like real world dogs. Which sometimes has the side effect of making the pups seem even younger.
Mixed-breed dogs that aren't designer breeds like Skye's cockapoo are looked down upon, similar to biracial humans.
Tied to the above, certain dog breeds are subject to Fantastic Racism that parallels their experiences in Real Life (like pitbulls and their mixes being ruthless bloodthirsty monsters, for example).
Rocky's Ear Notch is the result of abuse by a human (usually but not always with a knife) prior to him coming into Ryder's care. His fear of water also comes from similar Dark and Troubled Past elements.
Zuma has rhotacism (speech impediment making someone unable to pronounce R-sounds). This came about because his first voice actor Emily Thorne had it, and even though it was gone when Emily was replaced by Carter in 2017 it's remained an aspect of Zuma where a lot of fanfics will replace the Rs in his speech with Ws.
The villains, especially Sid Swashbuckle, with Sweetie and Mayor Humdinger to a lesser extent commonly depicted have a Dark and Troubled Past as an attempt to justify their actions.
Popular fanon is that Rico was dropped as an egg and this is why he is the least intelligent of the four penguins.
Julien being transgender has caught on due to him being a male lemur leader. The fanfic Princess (Laburnum Steelfang), where Julien being trans is a major plot point, also helped popularize the fanon.
Even though the show is confirmed to take place in an Alternate Continuity from the Madagascar movies, many argue that the events of the first and second movie are still canon, especially considering Alex's involvement in "The Revenge of Dr. Blowhole".
The penguins are brothers. This is never really made clear in the series, but is usually accepted in the fandom, even some fan wikias outright state it as a fact. Their movie officially goes with the idea of them being adopted brothers.
Any information about their missing biological parents is Fanon and unimportant as far as the creators are concerned. This includes the idea that Doofenshmirtz is Phineas' father, which has been actively jossed by the show's creators.
Also their ages, though the non-canonical pilot episode says they're 9. The episode "Quantum Boogaloo" seems to imply they're 10, but Word of God says this may not be exact. A common piece of fanon used to be that Ferb was older than Phineas, most likely because he's taller, has a deeper voice, is more subdued and actually expresses an interest in girls. Again, Word of God discounted that, saying the two are the same age. After an episode focusing on Phineas' birthday, they then clarified that he's the slightly older one.
Lots of fans believe that Ginger (the Asian Fireside Girl) is Stacy's little sister. This was eventually confirmed on the show.
Utonium is often depicted a half-white, half-Asian. Some fans even make him related to Samurai Jack since they look similar.
Quite a few fans think the girls see Ms. Bellum as a maternal figure, though Utonium is still shipped with Keane more.
There are disproportionate amount of fanfics about the characters as teens, however there's also quite a bit of fanon, especially in Dark Fics, that they cannot age due to being Artificial Humans. Other side-effects include being immortal and (especially in Blossom's case) being very precocious to the point of Creepy Child degrees. The Powerpuff Girls (2016) seems to run with the interpretation that they're at minimum slow aging. In that show they're vaguely between 8 and 12 but still look like they're 5.
Fans ignore the fact the Powerpuffs literally have no fingers, toes, or noses.
Buttercup is often given an individual superpower, much like Blossom's ice breath and Bubbles' ability to understand every language.
Buttercup as a trans boy or as bisexual.
That the girls wear dresses with a stripe in the middle. It's supposed to be a stylized belt. Even the official crossover MMO FusionFall made this mistake.
The Real Ghostbusters has a bit of this. For example, it's generally accepted that Winston was in Nam and Egon makes great cocoa.
All that's canonically known about Malinda's death is that she died in the hospital, that's it. However, the consensus among fans is that it was an illness as opposed to an injury, and cancer is a common one in fanfics. The line from "Chuckie vs. the Potty" about "It's the worst thing that's happened to me since my mom put me on the bottle" has led to speculation that Malinda put Chuckie on the bottle because she knew she was dying.
Fred/Daphne started out this way. Fans noticed that the two always snuck off togethernote Itself a bit exaggerated, given that many scenes had Fred, Daphne, and Velma clue-hunting together while Shaggy and Scooby were elsewhere, leading to speculation that there was something between them. By the late 1990s, canon material began rolling with that idea, and it's now a major part of the franchise.
Velma as a lesbian is a long-standing fanon. It comes from her not having any love interests in most incarnations, her being based off a lesbian, and her fitting several stereotypes associated with lesbians, and the fact that the Real Life actress Velma is modeled on, Sheila Kuehel, is openly gay. This became partially Ascended Fanon in Scooby-Doo! Mystery Incorporated, as the creators stated that they intended for Velma to be read as a lesbian and gave her a female Implied Love Interest. It became fully Ascended Fanon in Trick or Treat Scooby-Doo!, in which Velma has an explicit crush on a woman.
Shaggy and Scooby as stoners. Canonically, the characters are based on beatnik stereotypes, though parodies (most notably an episode of Harvey Birdman, Attorney at Law where Harvey defends them from possession of marijuana and DUI charges) frequently make jokes concerning this, with even the 2002 live-action film giving this interpretation a shout-out. Sometimes, only Shaggy is portrayed as a stoner, since Scooby is a dog, and other times the Scooby Snacks are speculated to contain drugs.
Many fans of The Simpsons believe the popular theory that Eddie the Cop is Ralph Wiggum's biological father, even though it's never been confirmed by the creators. While not officially canon, the theory is pretty plausible: Ralph physically resembles Eddie much more than he resembles Chief Wiggum (they even have the same distinctive haircut), and Eddie has apparently worked under Chief Wiggum for years—so it's likely that he knows his wife.
Due to their similarities in appearance, Allison Taylor is sometimes played with as a long lost or estranged daughter of Ruth Powers.
If it's a fanfic starring Squilliam Fancyson, expect him to be either quite sexually promiscuous or a god in bed. Possibly both.
There's no explanation for why Pearl (a whale) is the daughter of Mr. Krabs (a crab), but the two common fan explanations are that she's adopted or her mother was already pregnant when Mr. Krabs got together with her.
In the first movie, when all the decorations in Shell City are revived, among them is a male pufferfish. A theory is that he's actually Mrs. Puff's missing husband.
Marco being genderfluid or a closeted trans girl and sticking with the "Princess Marco" identity to some extent. Among other evidence, Marco spends most of "My New Wand!" in Star's closet wearing her clothes (albeit hastily wrapped around his body), and remarks that he'd love to be queen in "Mr. Candle Cares". Then in "Heinous", not only does no one bat an eye at Miss Heinous repeatedly referring to Marco as "she", but when Heinous forces Marco to do a retraction video he addresses the audience with "It's your girl, Marco!" There's also Glossaryck (who is hinted to have some ability to predict the future) regularly calling Marco "Margo".
There's a lot of evidence that Star Butterfly is autistic. She stims (bouncing up and down, chewing on her wand, flapping her hands, among other things) and repeats herself (known as palilalia) when she gets tense or excited, she tends to be Literal-Minded, and she has poor social skills (she's often Innocently Insensitive and has No Sense of Personal Space).
Star being bisexual isn't an uncommon belief due to her having several Ambiguously Bi moments.
Many fans, especially on Tumblr, believe that Tom is pansexual, both to ship him with whoever they want and because when he shows up in "Friendenemies", he is licking an ice cream fashioned in a similar manner to the Pansexual pride flag. In the show proper, he is Ambiguously Bi and has made quite a few comments towards or about Marco that can be taken either way.
Star's Mewberty wings are sometimes depicted as ticklish, or sexually sensitive, in steamy works.
Star being jealous of Jackie's proportions, to the point of feeling insecure about her own body. In the actual show, Star does hold some jealously towards Jackie, but only in regards to her being the target of Marco's affections rather than anything superficial.
"Gift of the Card" made it popular to believe that Marco once took ballet classes. Incidentally, Marco taking karate lessons in order to impress Jackie.
Janna is always interpreted as having all her actions towards Marco be the result of crushing on him since childhood, like how Marco himself had feelings towards Jackie. The show itself never makes the nature of her flirting and teasing explicitly clear. Those who go the extra mile to actively ship the two also have them be childhood friends, when it also isn't clear in the show whether they actually interacted much before high school.
Festivia the Fun, one of the past queens of Mewni and considered a legendary party girl, is popularly characterized as a depressed alcoholic and Stepford Smiler, due to the fact that she was raised believing that her predecessor Eclipsa had abandoned her to run away with a monster. She also had to rule Mewni through a very dark period of its history, so it's not hard for fans to believe she dealt with high amounts of stress.
It’s generally accepted that Eclipsa got her signature wide-brimmed hat from her uncle Jushtin, who wore a similar one.
Even with being gag-oriented and having Negative Continuity that usually means there's no solid canon to go by (or perhaps because of this), Superjail! has collected some fanon over the years:
The Warden doesn't seem to have any other name (that we know anyway), but there are fans that will insist on naming him "David", after his voice actor David Wain (who he was modeled after).
Most characters don't have their eye colors depicted (except for in some shots or close ups), but there are fanartists that like going with the idea of Warden having purple eyes to accentuate his unusual nature.
Jared's assumed to have been abused by his parents as a child or that he was adopted, to add to the idea that his entire life has been filled with incidents of horrible luck.
Lord Stingray appears to be a white human male beneath his suit in "Stingstress" (although he never takes off his mask), although a fanon concept known as "Lord Beast Ray" was previously popularized by a Stingray roleplayer and other fanartists. Instead of being a man, Stingray was depicted as a yellow-skinned mutant creature with a face exactly the same as his mask, and spikes protruding from his back. Even after the show's reveal, Beast Ray continues to pop up here and there in fanworks.
Other fanon about his unmasked appearance has him as either a wicked and attractive blond man with sharp teeth, or as an evil doppelganger of the Warden with RED glasses, spiked black hair, and the sharp teeth.
There's the assumption and theory by some fan authors that the Warden or Mistress (or both) may secretly have feelings for the other. And then there are those who theorize that the two are actually long-lost siblings who never got to know each other (and thus making that time they slept together to be even more dysfunctional).
Based off the mind screw ending to "Terrorarium", there were fans that assumed the Twins to be part of a completely identical species and that they might not be brothers, either that or that they were only two of MANY brothers. The fake flashback in "Hot Chick" (to illustrate their lie about their past) seemed to play with the idea, but it was ultimately Jossed in season 3, where the Twins were revealed to be the younger siblings to a group of Triplets, and the five apparently being the only children of their overlord father.
Though it was never specified who exactly birthed Jacknife's illegitimate son, the popular fanon is that it was Lady Jacknife from "Stingstress". This is also coupled with the idea that she could be related to him in some way.
All of the Supermarionation series produced by Gerry Anderson in the 1960s (Supercar, Fireball XL5, Stingray, Thunderbirds, and Captain Scarlet and the Mysterons) take place in the same universe and timeline. This originates from publicity material generated to promote the series, but was never established in on-screen canon by any of the shows. However, the common concepts of a World Government, a World Navy, and certain technologies permeate the latter three series, and the Zero-X spacecraft debuts in the Thunderbirds feature film and then becomes the vehicle by which Spectrum's Captain Black goes to Mars at the beginning of Captain Scarlet. This was canon in the comics based on the series (both 60's and 90's), in which certain strips from each series crossed over.
Ezekiel gets a ton of this, since his personality was largely undeveloped. Common elements include him being a great cook (canonically, his parents are both Home Ec. teachers), that he hates taking off his hat (based on the fact that he never took it off on the show, at least until he turns feral in season three), and that he is extremely religious (since his name has strong biblical connotations and his homeschooled background matches many real-life children of fundamentalist Christian parents).
Noah is gay, specifically for Cody, who in fanon is usually bisexual. Hey, it's not like the creators are doing anything to discourage this. Even fans who don't ship NoCo usually assume that Noah and Cody are close friends, mostly likely because they're both fan favorites and have enough similarities and differences to make good Foils. Seasons two and three seem to confirm the latter theory, portraying the two as amicable but also fond of making fun of each other. Ridonculous Race may have jossed Noah being gay by giving him a relationship with Emma, however, but the majority of the fandom still sees him as at least bisexual.
Noah's friend Owen is also seen as bisexual by many fans, due to the fact that he is the only male contestant who Justin's handsomeness has the same effect on as the females, as well as his Odd Friendship with Noah (who as mentioned above is often believed to be gay or bi) occasionally carrying strong Ho Yay vibes to it. It's not uncommon for some to depict Noah and Owen as becoming a gay couple as a result.
It's commonly assumed that Courtney's parents push her very hard to succeed, as a way of explaining her very driven and occasionallyunpleasant personality, as well as her extreme perfectionistic tendencies and obsession with success at any cost.
At one point during the "Team Gwen vs. Team Trent" war, someone on a forum suddenly declared themselves on "Team The Intern" (referring to the blond one) and it was soon noted that he looked sort of like Billy Idol. The fandom now knows him as Billy the Intern, to the point where it's been used on the show's official blog.
Quite a few works relating to Duncan have him and his father having a bad relationship which is the reason for his behavior. Sometimes it's just them arguing. Other times it is outright abusive. It helps that Duncan's online bio states his father is a police officer, which naturally clashes with Duncan's delinquent status. However, just as often, Duncan is depicted as having become a criminal out of spite for his father's mistreatment of him if the relationship was always negative.
On the other hand, Duncan is often depicted as being a bit of a Momma's Boy. It helps that he and his mother are implied to have a better relationship than with his father, as shown by her sticking up for him when his father asks if his parents even love him and Duncan wishing her a happy birthday when in jail after All-Stars.
Heather's family is often depicted in fanworks as extremely dysfunctional as part of a Freudian Excuse for her behaviour. This does have some support from official materials too, as Heather's online bio says she has an Annoying Younger Sibling who takes pleasure in tormenting her and an exclusive clip shows her parents celebrating her absence and deciding to sell all her thing.
Rodney and Scott are portrayed by some as being related, due to both of them being from the countryside and having similar red hair and freckles. Depending on who you ask, they're either cousins or brothers (since Rodney states he has seven brothers).
Similarly, Noah and Dave are believed to be cousins by a lot of fans, due to their similar appearances and personalities.
It's not uncommon for Jasmine to be portrayed as Aboriginal, due to being dark-skinned and coming from the Outback.
Fanwork often has Geoff and Bridgette living together, and since they're both avid surfers, their home is almost always near a beach. It's also generally assumed that Brody and Bridgette get along well instead of having a Friend Versus Lover situation - some even suggest a One True Threesome between them all (especially with the Ho Yay moments Geoff has with Brody).
A popular Ridonculous Race concept is that the Rockers are in a very low-key relationship. The theory arose from their physical affection towards each other and Rock's tendency to hold Spud's hand during running segments.
Many believe the same for the Police Cadets, Sanders and MacArthur (unless they ship the latter with Brody).
During the first episode of the show, Chris mentions a pair of ice dancers who lost the gold medal at the Olympics. Fans will always depict them as being Jacques and Josee from Ridonculous Race, even after Word of GodJossed it.
Trollhunters never confirms Nomura's full name in any canon sources (the main show, spin-off comics, tie-in novels, or sequel series). However, many fanworks give her the full name "Zelda Nomura".
Winx Club: Tecna is often referred to as half-android, even though the only legitimate hint to such is a few throwaway lines in the non-4Kids dub of one scene in an S1 episode: "I've not been programmed for dating." "I'm going to go switch myself off now." (Goes off to sleep).
If Caleb has a surname in a fanfiction, 9 times out of 10 it will be "Hart." The reason for this is not entirely clear, although Cornelia/Caleb shippers like the idea of Caleb having CH initials the same as her.
Many future fics featuring a married Cornelia and Caleb will have Caleb adopting the surname "Hale".
Elyon is attracted to Cornelia, at least on a subconscious level. This is because the two have a lot of Les Yay and Elyon's perfect guy, according to Cornelia, is basically a male version of her best friend Cornelia ("tall with long blond hair and blue eyes").
Wonder Pets!: Ming-Ming is sometimes speculated to have an unrequited crush on Linny, due to putting her wing around her in one episode and mistakenly thinking she was referring to her when Linny said some cookies were for "someone I love" in another.
A few fanfic writers made Rogue a fan of Anne Rice after seeing that she had a taste for vampire novels in the episode "Spykecam".
Some fanworks portray Kitty being a fan of both Twilight and Taylor Swift, due to her teenybopper personality.
Jean is often assumed to wear thongs, while everyone at one point is referenced as wearing a thong in fanfics she's the only one consistantly described as wearing them, (Funnily enough, the only character 'confirmed' as wearing thongs is Rogue, as goes a piece of artwork made by Evolution's head artist Steven E Gordon), and that her hair is dyed red, born from Tabitha's comment in one episode and the Fan Dumb wanting to insult her.
Kurt is assumed to cut himself in the darker, more angsty fics who see him as a Sad Clown.
Bobby is almost always depicted as a prankster, despite not actually pulling any pranks in the show, crazy shenanigans, but no pranks.
Jubilee is often made into a tomboy, despite not even dressing like one (wearing a feminine coat and boots) because of her status as One of the Boys (and possibly her appearance in the Tie-In comics in which she appears to be quite sporty).
Tabitha being the girl who Really Gets Around because she flirts a lot (a part of the double standard the Fan Dumb has towards the girls of the show), but it may just be a case of them being in high school.
While it's a common fan-depiction for the character across other X-Men incarnations due to the idea of her absorption powers affecting her mental state, the Evolution incarnation of Rogue is the only one where the fandom idea of her being bisexual is usually portrayed as being a natural part of her personality rather than influence from her powers, though it's possible this might mostly stem from the Evo version's goth/punk aesthetic and the stereotype of goth girls as usually being bi.
A minor one, but Toad/Todd's poor hygiene stems from him having toad-like skin that reacts poorly to regular soap, and the soap he needs to use being expensive. So him showering once a month is credited to him trying to make his soap last or save up for more.
In fanfics, it seems to be quite common to assume that Wally's parents are abusive. While this is true in other continuities, their few appearances (in the show and tie-in comic) and Word of Goddiscount this theory. Which is probably for purposes of contrast: Wally's co-stars have dead parents (Robin), supervillain parents (Aqualad, Artemis, Superboy), parents living on another planet (Miss Martian), and overprotective parents who have been possessed by the Helmet of Fate (Zatanna). Wally is the only member of the team to have a demonstrably stable, mundane home life with a boring nuclear family.
Robin taking off his sunglasses for his lover (whoever it is) is an extremely intimate gesture and usually reserved for someone he's comfortable enough with to have his first time with.
Red Arrow uses drugs in Invasion. This is due to the fact he used heroin in the original comics. The cartoon never references drug usage, though it has allusions to it.
Impulse (Bart Allen) claims that the Flash (never specifying which, but clearly meaning Barry) taught Barry's kids how to use their speed abilities and that Bart learned from them. It's established at the end of the episode, however, that before Bart traveled back, Barry died that very day and didn't live to even see his children be born (or likely even learn they existed), never mind teach them how to use their speed powers. Obviously, Bart could've just been lying about the Flash teaching them entirely, or Jay Garrick could've been the Flash that taught them, but fanfics have often preferred to believe that Wally was the Flash that taught Don and Dawn, since Wally's death very likely wouldn't have occurred when it did before Impulse traveled back and he could've long outlived his mentor. The reason for this is obvious: Wally did come out of retirement and became the Flash proper when Barry died in the comics, and many fans still wish Wally had become the Flash in the show, and it's a very fitting Master-Apprentice Chain that Wally would teach his own mentor's children. There is, however, no verification of this in the show proper, with Bart almost never discussing the Flash Family in the future beyond his introduction episode. It's entirely possible even Bart doesn't know himself who taught his father and aunt (if they were taught by anyone at all), as his knowledge of the future does seem rather full of holes, and at no point does he ever claim Wally died earlier than he was 'supposed to', though fans usually take it as a given that he did.
Also relating to Impulse, fanfics often write Bart as though he possesses a comprehensive knowledge of every character's future (even though if he did, it would be entirely unreliable given the changes he made). In the show proper, his knowledge of future events seems limited almost entirely to his own family and the Reach, and even then seems pretty incomplete and unreliable. The unlikelihood he could ever possess such vast knowledge given the ruined state of his future, where such information likely wouldn't be available, and where most of the currently active heroes would likely have died many, many years before he was born, is rarely ever considered or brought up.
◊ where Wakko claims to be of the "other" gender so that the siblings could all be allowed to board Noah's Ark (with only one animal of each gender being allowed on board), and some of the crew members like the idea of non-binary Wakko.
Fan Dumb wanting to insult her.
