Fanon / Reborn! (2004)

• A generally accepted headcanon amongst the fandom is that Yamamoto's mother is deceased given that she's never seen or mentioned despite Yamamoto's close relationship with his father. Likewise, it's often inferred that Hibari's parents are absent, or simply negligent.
• Kaoru is the only member of the Simon family not to have revealed his Earth Flame. People have thus theorized it to be either Ocean or Meadow, given that each Simon member has had the kanji for their Flame name in their actual name.
• A good number of fans in the fandom believed that Tsuna's hyper intuition and flame being sealed caused an imbalance which was the reason why Tsuna is so clumsy and Book Dumb.
• The headcanon of Hibari and Fon having familial relations with each other as an uncle or great-granduncle due to their physically similar resemblance.
• With the implication of the tenth generation being the reincarnation of the first generation of Vongola. A good number of fans believed that Chrome is the reincarnation of Elena as she's the only odd one out among the tenth generation and was slightly supported in canon with her close relationship with Mukuro, who was implied to be Daemon's reincarnation.
• Characters being influenced by their flames and such are popular headcanons in the fandom as well. In the series, flames are only defined by their properties and everyone uses them for fighting. Meanwhile, read one fanfic and you will see many detailed descriptions as to how a flame will define a certain character's personality and how it influences everything around them. Which does have some precedence in canon but not to the point where it influences their life.
• The idea that Lambo's last name is "Bovino" and Dino's last name is "Cavallone" are generally accepted as their real last names since Amano had never officially given them one.
• A good chunk of the fandom have very different interpretations on Kyoko’s character; her not being as naive as the storyline portrays her, or that she’s not as kind as she appears. of course, these headcanons are met with varying reactions.
• In the fandom, everyone seems to think that Reborn and the other Arcobaleno will age and de-age as time passes by. This probably stemmed from Lal Mirch who, after ten years, had become an adult once again. But that only happened because she possessed a corrupted pacifier and isn't really an Arcobaleno since Colonnello took her place. There is no real evidence that the other Arcobaleno will age and de-age over time. This may have also stemmed from Luce, Aria, and Uni who looked incredibly alike. They were thought to be the same person (Luce) but it was eventually revealed that Luce is Aria's mother and Uni's grandmother.
• Also, the—now-confirmed—theory that Reborn was actually a full-grown man in the form of a baby was generally accepted by many fans as being true.