Iron Man 3 co-writer Drew Pearce recently brought up an interesting idea regarding the introduction of mutants into the Marvel Cinematic Universe. "I would not be surprised if WandaVision is the way they start to bring in, for example, mutants," he told Collider. Though there's nothing to suggest Pearce has any insider knowledge, there may be some merit to the idea of using WandaVision to bring mutants to the MCU.
There are a lot of theories about how Scarlet Witch will bring mutants into the MCU, with many of them looking at 2015's House of M as proof. That storyline saw Scarlet Witch using her powers to alter reality and drastically diminish the number of mutants in the world following the loss of her children. As there aren't currently mutants in the MCU, a lot of theories posit a sort of reverse House of M, in which she creates mutants. Could something like that happen in WandaVision? It's certainly a possibility.
Although there's not a lot of information about WandaVision just yet, there's some stuff that seems like a safe bet. Wanda will have to, in some way, bring Vision back to life. Whether that means building a new body or reigniting the old one, this will likely wind up being Wanda's primary goal, at least early on. There are also reports WandaVision will work on redefining Scarlet Witch's powers. Presently, Wanda's abilities aren't always clear, and the full extent of her powers has yet to be discovered. WandaVision will likely take great strides to properly show just how powerful she can be.
Some of Scarlet Witch's most impactful moments have come at times of great stress in the comics -- with the aforementioned House of M -- and MCU. In Avengers: Age of Ultron, after witnessing her brother die, Wanda releases a surge of energy that literally disintegrates every Ultron drone in sight. In Avengers: Endgame, with Vision's death fresh on her mind, Wanda comes deathly close to killing Thanos, with Marvel Studios President Kevin Feige saying she could have defeated him.
Wanda has had to deal with a great deal of emotional stress during her time in the MCU, and there's going to be a lot on her mind heading into WandaVision. Adding to this could be Vision's return and the risk of losing him again. If Vision is restored to life, there's no guarantee he'll actually stick around. His connection to life might also be tenuous, which could prove taxing emotionally. In having to deal with this stress, and failing to do so appropriately, Wanda could use her powers to potentially, and accidentally, introduce mutants to the MCU.
Doing so would definitely require giving Scarlet Witch's powers more like those of her comics counterpart. This could also lead to a redefinition of her as a mutant, as she hasn't been able to be on in the MCU up until now. Although the character was eventually retconned in the comics to have been the product of experiments from the High Evolutionary, WandaVision could deal with Scarlet Witch's powers and finally define them as either magic or mutant.
Doctor Strange in the Multiverse of Madness will also likely come into play here, as Scarlet Witch is set to appear in that movie. The film will draw on the events of WandaVision, meaning they must have some impact on Doctor Strange's world. Even if WandaVision doesn't end up being the Phase Four entry where Scarlet Witch creates mutants, Doctor Strange 2 could end up playing a role in some major way, as she'll be dealing with the fallout of her Disney+ series.
However, WandaVision and Phase Four probably won't introduce mutants to the MCU, at least directly. Although Feige confirmed Marvel Studios is working on the Fantastic Four and X-Men, they're likely not going to appear until at least Phase Five. Still, WandaVision and Phase Four could -- and likely will -- do a lot to lay the groundwork for a future with the X-Men.