Warning: contains spoilers for Avengers #56!
Marvel's Wandavision Disney+ series was the source of many fan theories, but one of them actually came true in the comics - though perhaps not as the fans imagined. As the villain Mephisto begins to enact a plan throughout the Marvel Universe that involves his many variants, he naturally attempts to manipulate the many superheroes of Earth. In Avengers #56, he does just that - and not to the Scarlet Witch, but to another superhero from the Thor franchise - Jane Foster.
In 2021's Heroes Reborn crossover, the Avengers never met and the mantle of Earth's Mightiest Heroes was held by the Squadron Supreme, Marvel's pastiche of DC's Justice League. The entire world was a ruse: Agent Coulson (the president of the universe) struck a deal with Mephisto to alter reality, but Blade gathered the Avengers and changed reality back to normal, defeating Coulson and sending the Squadron Supreme packing. Unfortunately, Mephisto revealed this was only a "trial run" - and now he attempts to alter reality a second time...with a nigh-infinite army of Mephisto variants from every corner of the Marvel Multiverse.
In Avengers #56, written by Jason Aaron with art by Javier Garrón, Jane Foster wakes up in a picturesque Asgardian house, the happy housewife of the mighty Thor. Occasionally, she feels uneasy, but she spends her days reminding herself that a content life of domesticity is everything she ever wanted. Unfortunately, the entire living situation is actually an illusion created by Mephisto (who manifests in the form of monstrous pigs with blood-red eyes). Mephisto retreated when Jane Foster from another universe comes to her counterpart's aid.
One of the most popular Wandavision fan theories centered around Mephisto, who many fans believed was in control of the Scarlet Witch and the entire town of Westview. This fan theory is actually true in Avengers #56, but it applies to Jane Foster, who is actually caught between two temptations: a life with Thor and a life as Thor, wielding Mjolnir and possessing tremendous power. Once free of the initial illusion, she even wonders if the real test of temptation is what happens after Mephisto's trickery...or is that what he wanted all along?
This is also effective for another reason: the Scarlet Witch has held power most of her life, and has been one of the most powerful members of the Avengers for decades. Jane Foster, by contrast, is a human who was granted powers only recently (in comic book terms) and would be fare more susceptible to demonic trickery than Wanda. Wandavision's most popular fan theory, in actuality, had very little to do with Wanda herself, and everything to do with Jane Foster and Mephisto.