Due to her penchant for Gratuitous Spanish as well as Adam proving ascended humans can become Angels, it's believed Vaggie was a Salvadoran human at one point like her original concept suggested, but became an Exorcist after dying and going to Heaven. Similarly, Adam "naming" her is believed to be her either never having a name in life, or discarding her identity as a human due to having such a shitty life.
Angel Dust being a veteran of World War II is often used as a neat way of more thoroughly explaining his proficiency with guns beyond his gangster backgroundnote Make note of the fact that Angel's primary weapons are Thompson SMGs, which while primarily associated with Prohibition-era gangsters in their drum-mag form saw a lot of action in the European Theater as a standard-issue American weapon.
Some fans believe the Egg Boiz who died with Sir Pentious during his Heroic Sacrifice also made it into Heaven despite no evidence suggesting an ascension.
On that note, the Sole Survivor of the Egg Boiz is assumed by many to be Frank.
While not universal, it's not uncommon for the cyclopean Cherri Bomb's living human form to be drawn with only one eye as well, either wearing an eyepatch over the other one or just keeping it permanently closed, suggesting she became a cyclops demon through Morphic Resonance.
Charlie
Charlie having a Covert Pervert side is extremely common. It generally relates to her relationship with Vaggie, but it can deal with her getting a little too interested in other characters' sex lives.
Some took it a step further and said that Charlie is a succubus, seeing as her mother is canonically Lilith. This was later apparently jossed when succubi and incubi were shown to be their own class of demon in the Hazbin/Helluva version of Hell, ranking only second to last in the demonic hierarchy.
Many fans believe that when Charlie got so mad at Adam for killing Sir Pentious that she went all full demon mode and fought Adam to the point that she did what none of the others did and physically hurt him, despite all that, the fans believe that Charlie was still holding back. If that is true and Charlie decided to use her true final full demon mode, then Lucifer wouldn't have needed to fight Adam at all.
Alastor
Alastor constantly attempting to eat Fat Nuggets, Angel's pet pig.
Alastor having a deer tail that he hides under his coat, probably because he thinks it's too "cute" for someone like him.
Portraying Alastor as having a Paralyzing Fear of Sexuality due to his confirmed asexuality and his startled reaction to Angel making a pass at him.
Niffty
Many fans speculate that Niffty is Alastor's daughter/sister or somehow related to him since he treats her the best out of the cast.
Considering that Niffty was a housewife in the 50s, many fans have theorized that she was subjected to lobotomy and that's why she is they way she is and has a fascination with needles.
Niffty being Japanese/a yokai because her original voice actress is fluent in the language and has even spoken Japanese in Niffty's voice during a livestream. Her voice actress in the show proper being half-Japanese probably helps this idea.
Niffty's overexcited tone when asking if there are any men in the hotel has led fans to believe she is a Yandere and her sin involves inflicting violence on her love interests, or having killed her husband. This was more or less confirmed canon in the series proper.
Husk
Fans have decided that if anyone acts like the animal their demon forms are based on, it's Husk. His original voice actor even voiced him purring during one of the livestreams. To a lesser extent, Vaggie being attracted to bright lights sees a decent amount of fanart as well.
Many fans interpret Husk as having a Deal with the Devil, which is why he's not afraid to shove, curse, or talk back at Alastor. It also explains why he was so easily swayed with Cheap Booze and why he asks what Alastor wants with him "this time". In a case of I Knew It!, this turns out to be exactly what happened, as revealed in "Masquerade"
It's generally agreed that Tom Trench died in World War One, given his surname, gas mask, and the fact that his official year of death is during WWI (Tom died in 1918, the war ended that November). Though he was originally going to have died in the Chernobyl disaster.
After "Addict" revealed what an abusive prick Val is to Angel, fans often speculate that Val would gladly cook and feed Fat Nuggets to Angel over lunch as a power move if given half the chance. The fact that, according to Instagram posts, Val bought a pet himself and almost immediately shot it to death because it annoyed him only feeds this theory more.
For fans who don't ship Zestial and Carmilla together, some interpret both as a gay man and a lesbian respectively who simply have a strong platonic bond and solidarity.
Heaven
The Archangel Michael is often portrayed as being the borderline identical twin brother of Lucifer Morningstar, due to a popular fan-interpretation of what he would look like in the Hazbin Hotel-verse. On a related note, it's common for Lucifer to be depicted, including on This Very Wiki, as God's son, a relationship popularized by Supernatural and Lucifer (2016).
There's a theory based on scripture that humans who make it to Heaven undergo Laser-Guided Amnesia to forget any friends or family who were sent to Hell, meaning Molly wouldn't recognize her brother Angel Dust and Sir Pentious will have forgotten everyone from the hotel. This tends to ignore that Adam has memories of Lilith still, even making a deal with her.
A rather common belief seen is that there are a group of "Elders" in Heaven that even Sera has to answer to. This belief largely stems from the depiction of the Angels in Charlie's story in "Overture", due to their resembling "Biblically Accurate" Angels, and that the Angels seen in "More Than Anything" that Lucifer tries to shield Charlie from are meant to be them, running contrary to the show stating Sera is the leader and therefore highest authority in Heaven, with her even being seen among the so called "Elders" in Charlie's story overseeing the Earth.
A semi-common belief is that "Sera" is actually an In-Series Nickname, and that her full name is "Seraphiel", stated in Abrahamic texts to be "Chief of the Seraphim" , one of the "Eight Judges" of Heaven, and possessing the "face of an Angel, body of an eagle", fitting with how Sera is leader of Heaven, took the role of Judge when presiding over Charlie's case for Sinners being redeemable, and her briefly seen true form looking avian.
Fanarts depicting Adam and Emily interacting will often portray Adam acting as a Big Brother Mentor / Cool Uncle / babysitter or trying to behave himself, especially in the presence of Sera.
Many fans agree that if Emily the Seraphim were to become a fallen angel because of her willingness to help Charlie's people, despite being against Sera's wishes, that Emily will remove her halo herself, voluntarily making herself into a fallen angel.
Due to the fact that she seems way younger than the other Seraphim and that her role to bring joy to the souls of Heaven, many believe that Emily was created as a replacement for Lucifer after his banishment to fill the role he previously occupied as a dreamer.
The sins seeing each other as siblings was canonized in Helluva's "Queen Bee", and Belphegor's name is in Charlie's contact list in "Dad Beat Dad", which is probably the closest this'll get to being confirmed canon since characters from the two shows legally can't interact.
Some fans believe that back at the Garden of Eden, Adam and Lucifer were friends or at the very least were on good terms, until Lucifer seduced Lilith.
For that matter, the idea Lucifer "seduced" Lilith at all, as Charlie's story, if taken at face value, suggests Lilith left Adam before meeting Lucifer, and the two fell in love on their own.
Some fanart of Eve depicts her with black or brown hair to differentiate her from Lilith.
