It's no secret that WandaVision will tie directly into Doctor Strange in the Multiverse of Madness, with Elizabeth Olsen having been confirmed to reprise her role as Wanda Maximoff way back in 2019 when the movie was first announced. While the Sorcerer Supreme himself didn't show up in Westview as was theorized by many fans watching WandaVision, the finale still laid the groundwork for the 2022 movie.
MAJOR SPOILERS AHEAD FOR WANDAVISION EPISODE NINE!
Once again, Wanda was forced to say goodbye to Vision in episode nine, and this one may have been the most heartbreaking of them all. As the titular couple watched the boundaries of the Hex slowly close in around them, Wanda knew that it was the end of her love, and also her two children.
However, the finale's second post credit scene revealed the Scarlet Witch using her astral form to scour the Darkhold, looking for a way to bring Tommy and Billy back for real. With a whole new level of power at her fingertips, there is nothing that Wanda won't do to bring her boys back to her, including but not limited to breaking the natural laws that Doctor Strange is sworn to protect, and potentially opening the multiverse.
As Agatha told her during their battle above Westview, she has no idea what she has unleashed. Wanda's power is all but limitless, as proven by how easily she was able to defeat Agatha, and is greater even than that of the Earth's Sorcerer Supreme.
Since taking on the title, Stephen Strange has kept a list of powered individuals and beings that pose a potential threat to the world, and if Loki is deserving of a place on said list, then the Scarlet Witch certainly is. She will likely have grabbed Strange's attention when she created the Hex, as she did Agatha's, and if he is to put himself between her and the dark magic she is using to try and bring back the twins, she is likely to test the theory that her powers trump his.
Even her position while studying the Darkhold is reminiscent of how Strange himself was able to use his astral projection at Kamar-Taj, the only difference being that Wanda doesn't have to be asleep to project herself.
And if you needed further proof that this post credit scene was a clear set up for the Doctor Strange sequel, Michael Giacchino's score from the first movie can be heard in the background behind the screams of Wanda's children.
Though it's unclear right now if the Scarlet Witch's role in the Multiverse of Madness will be strictly villainous, WandaVision went a long way to teasing the clash of two of the most powerful heroes the MCU has ever introduced.