WARNING: The following contains spoilers for WandaVision Episode 5, "On a Very Special Episode...," now streaming on Disney+.
The best thing about WandaVision being a weekly MCU show is that fans can debate and speculate about what happens each week, making the upcoming episodes can't miss television, and Episode 5 is extra special with, as Darcy puts it, a recasting of a Marvel Cinematic Universe character who's been dead for years. At the end of the episode, Wanda answers the door and sees her brother, Pietro Maximoff. However, this is not the Pietro the MCU knows. Instead of Aaron Taylor-Johnson, the actor who played Quicksilver in Avengers: Age of Ultron, it is Evan Peters, the actor who played Quicksilver in the X-Men movies by Fox.
The debates raged on about whether this was really Quicksilver from the Fox world at all. Many speculate he is just another person trapped in the town, recast to play Quicksilver since Wanda herself said she couldn't bring back the dead. He could also be the missing person from witness protection Agent Jimmy Woo is searching for. However, there is also the theory that this is Quicksilver from the Fox universe, and it's Doctor Strange who brought him here.
While Monica Rambeau has tried her best to figure out what is going on in the town, S.W.O.R.D. seems to have its own agenda. If S.W.O.R.D. is anything like S.H.I.E.L.D., the organization will always attack a problem from different angles, and if Director Tyler Hayward is anything like Nick Fury, no one should trust him to be completely honest. He could easily have Monica, Jimmy Woo and Darcy Lewis hitting things from outside the walls while also working with someone, like Doctor Strange, on another angle to get into Westview.
With the Ancient One dead, Doctor Strange is the Sorcerer Supreme, which means his knowledge of the universe is almost limitless, and that is normal for magical beings in his esteemed position. For instance, in Avengers: Endgame, when Hulk goes to get the Time Stone, he meets the Ancient One. She's unwilling to give it to Hulk and mentions that he is from "a timeline," but not necessarily her timeline, so it's clear she knows about the multiverse, the different timelines and how new timelines are created. If the Ancient One knows about the multiverse, there is no doubt that Doctor Strange also knows about it.
Pietro Maximoff was around in at least two different versions of Earth, and there is a chance that Doctor Strange is trying to figure out how to help Wanda. By bringing in a version of her brother, one he plucked from another Earth, he could help soothe her pain and suffering, which seems to be why she created this idyllic, sitcom world.
It is also important to remember that Agnes was there when Wanda talked about not bringing anyone back from the dead. If Agnes is Agatha Harkness, there is also a chance that she has made contact with Doctor Strange somehow, and the two of them are working together to try to free this town. With Wanda expressing how she isn't open to bringing the dead back to life, brining in a different Quicksilver may have seemed like a safer choice for Agnes and Strange, if this is the case.
Doctor Strange has always seemed to be there to help Wanda in the comics. He was there to shut her down after the "Avengers Disassembled" storyline, which saw Agatha Harkness' death, as well as the deaths of Hawkeye and Ant-Man. More recently, he was there to stop an army of zombie mutants when Wanda tried to bring back everyone who died in Genosha to horrific results. With Wanda appearing in Doctor Strange in the Multitude of Madness, it wouldn't be unheard of to see him working behind the scenes of WandaVision to help clean up this mess, whether he is doing it with S.W.O.R.D., with Agnes or by himself.
Written by Jac Schaeffer and directed by Matt Shakman, WandaVision stars Elizabeth Olsen as Wanda Maximoff/Scarlet Witch, Paul Bettany as Vision, Randall Park as Agent Jimmy Woo, Kat Dennings as Darcy Lewis, Teyonah Parris as Monica Rambeau and Kathryn Hahn as Agnes. New episodes air Fridays on Disney+.