There are theories about Azura from the books Luz like actually being real and how Luz is related to her in some way. "Lost in Language" reveals that Amity is a fan of the series like Luz is, meaning that the Azura books aren't just something found on Earth. Moreover, it's noticeable that most of Azura's ears are covered by her hair — pointed ears are a sign of someone from the Boiling Isles — so whether or not she really is a true Witch or not is a mystery.
Fans have spelled out "A WITCH LOSES A TRUE WAY" with the first letters of each episode title in Season 1, leading to theories of what that could possibly mean. It turns out to refer to Luz and Emperor Belos losing the portal to Earth and Eda and Lilith losing their magic.
Each episode contains a secret hidden word; all the words from each episode of Season 1 spell "Two witches torn apart, now alone, two hearts of stone, a curse of feathers and mud, a betrayal of blood."—turns out to be very fitting considering "Agony of a Witch" reveals that Lilith was the one who cursed Eda...
Because of Amity's hair being dyed green to match her siblings, as well as her overachieving personality, it was commonly speculated that she was an illegitimate Chocolate Baby attempting to compensate for being The Unfavorite. Season 2 seemingly deconfirms this, showing that she inherited her brown hair from Alador, with Word of God later stating that Odalia makes her dye her hair green for Control Freak reasons.
Early on, several fans suspected Luz was simply imagining the whole show on her way to camp, though this died down once it became clear how much the show focuses on subverting the classic fantasy tropes she loves so much (plus that kind of ending working much better in a more short-form story where it wouldn't be rendering potentially years of development meaningless).
Upon the first appearance of Philip Wittebane, the first human to discover the magical world, quite a few fans suspected he's actually Emperor Belos. Despite having different voice actors, they sound similar enough to make an easy connection. There are also those speculating that Philip is an ancestor of Hunter, due to the fact that Hunter can't perform magic normally despite being called a witch."Elsewhere And Elsewhen" reveals this to be true.
After the charity stream had Dana Terrace draw a sketch of the upper half of the Collector's face and called it a character the audience had "sort of already met before", some fans took note of their appearance being quite similar to Luz, leading to a theory that the Collector would either possess or otherwise mimic Luz's form. This caught further on with the release of "Hollow Mind", which revealed that the Collector doesn't have a proper physical form (or it is sealed beneath the Titan's bones), instead appearing as a shadow on the wall, and working with Belos to be freed, possibly by possessing a vessel like Luz. This gained even further traction in "Clouds on the Horizon", where the Collector talks about getting a "third player" and Luz is taken to Belos after switching with Hunter. The theory was ultimately jossed, with the similarities between the two being more about the Collector being Luz' Mirror Character.
"Yesterday's Lie" confirms that two brothers were taken from the Human Realm to the Boiling Isles during the 1600s. Almost immediately, many declared that one of them was Philip Wittebane while the other is speculated to actually be Emperor Belos. It helps that a picture of the two shows a cardinal, the same animal that would become Hunter's Palisman, perched on one's shoulder, adding to the theories that Hunter is Philip's descendant.
Due to the background photos/memories shown in "Hollow Mind," the idea that Philip/Belos's brother married an ancestor of the Clawthornes is one theory that is slowly rising in popularity since the witch on one of the photos has a strong resemblance to Eda and Lilith's mother Gwendolyn. This would make Belos, and possibly Hunter, a distant relative to them and would make the former's hatred and connections to the Clawthrone clan even more serious than it already is.
When Belos was first introduced, his voice actor, Matthew Rhys, stated that he was "misunderstood", prompting speculation that Belos would have redeeming traits, which ended up not being the case and then some. When the sheer depth of his evil was revealed, fans speculated what Rhys meant by his statement. One was that Belos was originally planned to be relatively more sympathetic, something that could be inferred from his first appearances, or that Rhys simply was just as kept in the dark of Belos's true character as the fans, or that Rhys was referring to how he approached the character's mindset (since Belos believes himself to be the hero and couldn't fathom why Luz would see him as a monster). Some have semi-jokingly proposed that it was actually referring to how the fans were misunderstanding Belos as a villain with any sympathetic traits.
