WandaVision is the Marvel Cinematic Universe's first real deep dive into its titular characters. Neither Wanda nor Vision ever had a standalone movie; they didn't even appear outside of the crossover films (unlike Black Widow and Hawkeye, who appeared in Iron Man 2 and Thor). That makes this the first chance the franchise has had to dive into the Maximoff sibling mythology. But if they do, there's a catch: Which Quicksilver will the show use? With Fox's X-Men now part of the MCU, will Evan Peters play Pietro in WandaVision? Or will that role go to his Marvel counterpart, Aaron Taylor-Johnson?
Warning: Spoilers through WandaVision Season 1, Episode 3 follow. By the early 2010s, Disney's MCU was a massive success... and a mess, with the rights to major franchise characters sold to other studios. Spider-Man was owned by Sony; X-Men was controlled by Fox. In the case of the latter, this meant the MCU had to rewrite certain mutant characters out of X-Men lore before using them in films.
This led to the paradox of two Pietro Maximoffs appearing on the big screen within weeks of each other in 2014. In Fox's version, he appeared as he is in the comics: Quicksilver, an X-Men mutant with super speed, played by Evan Peters. In the MCU, Pietro was played by Aaron Taylor-Johnson in Avengers: Age of Ultron. There, he and his sister, Wanda (Elizabeth Olsen), were changed to a pair of Sokovian-born villains whose powers resulted from HYDRA's genetic alteration experiments.
The MCU dealt with this accidental doubling by killing Pietro off in Ultron. From then on, the MCU had Olsen's Wanda, and Fox had Peters' Pietro. That remained until 2019, when Disney bought Fox, giving Marvel back the X-Men characters.
It's not clear when new X-Men content will arrive in the MCU. However, it's widely believed within the fandom that, outside of Ryan Reynolds' Deadpool, most of the Fox lineup of super-beings will be recast. But that hasn't stopped fans from hoping otherwise.
Now, rumors are rampant that Evan Peters' Quicksilver will show up in WandaVision, thanks to Monica Rambeau breaking her cover as Geraldine and bringing up Pietro's death. Wanda already seems to be using WandaVision's odd version of reality as a repudiation of the real world, like when she declared Vision has an indestructible head. It would not be out of place for her to have Pietro show up for a visit to prove he's alive and well in her reality.
But since this is a fantasy in Wanda's head, there's no reason to bring back Taylor-Johnson. Peters is right there and was still playing Quicksilver as recently as 2019's Dark Phoenix. Moreover, "switching actors in main roles and no one saying anything about it" is a longstanding sitcom trope. Having Wanda's brother show up, played by the actor from the other version of the franchise, would be the ultimate reference within a reference within a reference.
Rumors Peters would be playing a "mysterious role" in WandaVision have been bouncing around since the spring of 2020. If it turns out they are correct, WandaVision fans are in for a treat. It would also give Disney+'s first Marvel show a Baby Yoda-level surprise reveal, that the X-Men are already here.