WARNING: The following contains spoilers for WandaVision Episode 5, "On a Very Special Episode...," now streaming on Disney+.
WandaVision's "On a Very Special Episode..." lived up up to its title by bringing in Evan Peters as Pietro Maximoff. Peters was Quicksilver in Fox's X-Men franchise, making him the first actor from that cinematic universe to cross over into the Marvel Cinematic Universe as a potentially new version of his character; however, who this Pietro is remains unclear, and there is a possibility that Peters' character isn't even Pietro; instead, he could be another citizen of Westview.
In this new reality, Westview residents have had their minds altered, forcing them to take on new personalities and names. Episode 4, "We Interrupt This Program," proved this, with S.W.O.R.D. uncovering the true identities of several characters. There have also been moments throughout the series that show Westview residents either don't know what's happening or they are aware of the fact that they've lost their free will, as seen in Episode 5.
As Vision taps into Norm's mind, Norm becomes his true self, and he is distraught over the fact that someone else is forcing him to be a new person. With many Westview citizens likely in the same position, it wouldn't be a surprise to learn that Pietro is another resident who's being forced to play a role in Wanda's suburban fantasy.
After all, he is clearly not the Pietro from the MCU. Darcy Lewis acknowledges this, remarking that Wanda recast her brother. The MCU has had a handful of characters recast before, like James Rhodes and Bruce Banner; however, the films have not directly addressed this. WandaVision, on the other hand, calls out the fact that this Pietro is different from the original MCU Pietro, played by Aaron Taylor-Johnson. While this could be another moment of Darcy representing the viewers, acknowledging in-universe that this is a different version of Wanda's brother could mean he isn't even Pietro.
Instead, he could be a resident who hasn't been featured in this sitcom until now. As seen with Monica's Geraldine, not everyone starts off as featured character, but they may eventually earn a chance to step into the spotlight, developing a name and personality that suits Wanda's fantasy. Having someone play her brother wouldn't be out of the question, especially since she was grieving over Pietro in Episode 3 and talking about him to her sons in Episode 5.
Even Wanda is shocked by his arrival, which further supports this theory. Wanda could simply be surprised that her dead brother is alive; however, it isn't until Pietro says he is her brother that Wanda accepts this as fact. Perhaps she didn't recognize him because he was formerly one of the residents Wanda had yet to encounter until now.
While the fake sitcom has a core cast of named characters, the town of Westview is bigger than that. It has a population of 3,892 people, and only a handful have had speaking roles in this sitcom reality. The few featured residents are present because they serve Wanda and Vision's narrative, and with the role of Wanda's brother open, a new resident needed to take that position, having their identity altered to suit the sitcom's story.
Furthermore, it's established in Episode 5 that everything in Westview is real, and Wanda has been changing reality, but she needs a physical being or object to work with. For instance, she needed Monica's bullet-proof vest to make into period appropriate clothing. In a more extreme case, she needed Vision's body to have him alive in Westview. While it's unclear how Vision was resurrected, this principle could also apply to Pietro.
Given how long it's been since Avengers: Age of Ultron, Pietro's corpse is likely unaccessible to Wanda, so she couldn't bring him back. While someone else could be influencing Westview alongside Wanda, it's likely the same rules apply to them. Without Pietro's body, a Westview resident was needed to bring Quicksilver back into the MCU.
On the other hand, WandaVision has shown that living beings can be created within Westview, as is the case for Billy and Tommy. This could be the case for Pietro as well, with the mysterious force behind Westview creating him to appease Wanda. However, if he was created out of nothing, then he would've likely resembled his past self. Furthermore, if the show wanted this to be another case of the MCU simply recasting someone without changing who the character was, then it would've been best not to directly mention the recast in-universe.
WandaVision continues to be one of the MCU's more complex properties, and the introduction of Peters' Quicksilver is proof of that. His presence raises questions about the multiverse and mutants; however, he could simply be another Westview resident forced into a new identity, but nothing is simple in WandaVision.
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+.