The majority of MCU Phase 4 projects have largely stood on their own, at least alongside the other post-Avengers: Endgame content that has been released from Marvel Studios. The exceptions have been the Spider-Man movies telling an ongoing story. That's about to change, though, as the Scarlet Witch - last seen in WandaVision - will play a key role in Doctor Strange in the Multiverse of Madness.
While Yelena's trajectory from Black Widow to Hawkeye was similar to how Infinity Saga characters migrated from the movies to Disney+ projects, Wanda Maximoff going from WandaVision to Multiverse of Madness will be the first time major character has appeared in a movie after playing a significant role in a Disney+ series.
This means Marvel fans are going to find out just how much the Disney+ shows will affect the movies that come after them. So far it seems as though the assumption from the folks at Marvel Studios is that viewers of the Disney+ shows are caught up on the movies, but not necessarily vice versa (likely due in part to the unavailability of the service in certain areas). WandaVision turned out to be a major hit, though, and its story contained a lot of key moments for the Scarlet Witch that could come into play in Multiverse of Madness.
Wanda Studies the Darkhold
In the post-credits scene of WandaVision's final episode, Wanda can be seen astral projecting as the Scarlet Witch reading the Darkhold - a book of magic which she took from Agatha Harkness before leaving Westview - while alone in a cabin at Mount Wundagore.
Before flying away from Westview to Mount Wundagore, Wanda states her determination to gain an understanding and stronger level of control of her powers, which she is likely trying to do by reading the Darkhold. Unfortunately, as fans of Marvel comics know, the Darkhold is a dangerous book that can affect the reader in negative - and potentially devastating - ways.
While reading the Darkhold, Wanda hears the voices of Billy and Tommy, who disappeared along with Vision and the rest of the Hex. Now that she knows they're alive somewhere, it's likely her next course of action will be to try to find them. It can probably be assumed she'll use what she's learned from the Darkhold in order to achieve this new goal. But whether it will ultimately end up helping or hurting her, in the end, remains to be seen, and will likely be central to her arc in Multiverse of Madness.
Wanda's Warning to SWORD
In one of the most shocking moments of Phase 4 thus far, Wanda steps out of the Hex in the fifth episode of WandaVision in order to remove the drone sent in by SWORD, issuing the organization a stern warning to stay away from her new home. As a part of said warning, she manipulates the minds of the SWORD agents with their guns pointed at her to turn them onto organization director Tyler Hayward, who was particularly hostile towards her.
If Wanda really does end up on a mission to find her children in Multiverse of Madness, it's possible - even likely - she'll be similarly ready to take down anyone who stands in her way, and now that she's learning more about the potential of her powers, her wrath could be much more severe this time around, especially if she ends up taking on someone like Doctor Strange himself.
Billy and Tommy Maximoff Age Themselves Up
At two - nearly three - different points in WandaVision's fifth installment, "On a Very Special Episode...", Billy and Tommy age themselves up a few years. At the end of the episode's opening scene, they go from infants to young children, and a little while later they grow to ten years old.
While Wanda stops the pair from aging up yet again after the death of their beloved dog, Sparky, it's possible that they could do so during or even prior to the events of Multiverse of Madness. With a Young Avengers project rumored to be on the horizon, Speed and Wiccan would seem like natural inclusions for the team, and they'd need to be at least several years older than they were in WandaVision in order to be in a similar age range as the other rumored recruits like Hawkeye's Kate Bishop.
White Vision Flies Away From Westview
After a philosophical discussion on the Ship of Theseus thought experiment during the WandaVision finale, Hex Vision makes White Vision regain the memories of the original Vision. The latter then flies away to an unknown location.
This, combined with Hex Vision and Wanda discussing the possibility of meeting again in the future despite the latter's imminent destruction at the end of the episode, indicates that Vision will probably return to the MCU again in some capacity, likely reuniting with Wanda once again at some point.
So, will this happen in Doctor Strange and the Multiverse of Madness? Quite possibly, as it seems like any Marvel character is fair game to appear in the movie, and Scarlet Witch will be one of the film's central characters. Like her use of the Darkhold, the reappearance of Vision is something that could help or hurt Wanda; he could help pull her out of any trouble the Darkhold gets her into, but the last time she was desperate to keep him around, she held an entire town hostage.
If Vision doesn't end up appearing in or reuniting with Wanda in Multiverse of Madness, it may be a case of Marvel wanting the two of them to have their own adventures before seeing each other again. Perhaps Vision will appear next in a project like Armor Wars, but it's certainly possible an appearance in the Doctor Strange sequel could be in the cards.
The Nexus Commercial
The seventh episode of WandaVision featured a commercial for the fictional product Nexus, which appeared to be an antidepressant-type medication. While the rest of the fictional products advertised in other WandaVision episodes served as references to either Wanda's past or present, this one didn't end up having much meaning beyond perhaps her return to reality from the Hex at the end of the series.
However, another MCU project ended up explaining the true meaning of "Nexus"; the Time Variance Authority in Loki was working to prevent Nexus events from causing timeline branches to emerge, thus leading to a new Multiversal War.
This certainly seems like something that will come into play in Doctor Strange and the Multiverse of Madness, especially since Loki ended with the very unleashing of the Multiverse itself. Perhaps we'll hear about certain Nexus events that occurred, or in Wanda's case how she might use her magic to deliberately create them in order to branch into other realities.
Chaos Magic and Scarlet Witch's Destiny
WandaVision reveals a lot about Wanda's powers, including that she didn't actually get them from the HYDRA experiments she participated in, presumably having had them all her life. At the end of the show's eighth episode, Agatha reveals that the powers Wanda is using to create and maintain the Hex - which viewers saw firsthand via the flashback to its formation earlier in the episode - are classified as chaos magic, and in Wanda's case they allow her to maintain many strong spells simultaneously and without even consciously thinking about them.
It's very likely Wanda will get to use this level of power again in Multiverse of Madness, especially after studying the Darkhold in hopes of understanding her abilities better. However, things may take a turn for the worse in this regard, because in the WandaVision finale Agatha says that as the Scarlet Witch, Wanda's destiny is "to destroy the world".
This phrase could be interpreted a lot of ways (in fact, the original plan was for it to come to fruition with the removal of the Hex) but it's possible Wanda really will destroy the world, or at least a world, in the Doctor Strange sequel. If this were to happen, it would most likely be the destruction of an alternate universe, so as not to affect the overall storytelling of the MCU, but it would still be a huge deal for Wanda to cause that much damage with her powers.
All nine episodes of WandaVision are available to stream on Disney+, and Doctor Strange in the Multiverse of Madness is set for a theatrical release on May 6.