Agatha Harkness is returning to the MCU, according to Marvel Studios' Kevin Feige. The breakout villain of WandaVision certainly made an impression in her role as the Big Bad of Westview, but her future was somewhat uncertain until the revelation that she will be coming back and "soon."
Her return to the MCU is not too surprising, given the direction of the franchise as well as Agatha's deep roots to numerous teams and characters in Marvel Comics lore. She could appear in any number of places going forward and is likely to be somewhat of a major player in the next phase.
Doctor Strange In The Multiverse Of Madness
Agatha could very well appear in Doctor Strange in the Multiverse of Madness. She has a strong connection to Wanda Maximoff, who will appear in the sequel, as well as chaos magic, which Marvel Comics fans know fuels Wanda's power.
The multiverse is obviously going to play a major role in the story, but so is magic, so Agatha appearing here makes the most sense. If the Scarlet Witch becomes the powerful threat Agatha warned about in the season finale of WandaVision, it may take Agatha to combat her.
Loki Season Two
Magic is also a big part of the Loki series. Magic counts among some of Loki's greatest powers, and Sylvie, too. If season two of the series delves deeper into that aspect of the characters, Agatha might make sense as a player in the story.
She wouldn't necessarily have to be the Agatha from the present or even the MCU timeline. She could be a variant who has followed a different path in life, or Loki could visit her past in Salem, where Agatha's exploration of dark magic led to the death of her entire coven.
What If...?
A likely place for Agatha Harkness to show up next is the upcoming animated What If...? series. This anthology series will explore alternate realities where things went differently for the timeline, like Peggy Carter receiving the Super-Soldier Serum.
Agatha could appear in this series in a number of ways, either as a role player in an episode focusing on Wanda or Vision, or perhaps even the Fantastic Four or Avengers. Agatha is associated with both in the comics and could be more so in a divergent reality in the animated series.
Ant-Man And The Wasp: Quantumania
A lingering mystery of WandaVision is who was the missing witness FBI agent Jimmy Woo trailed to Westview? Was it Agatha Harkness? If so, she could make an appearance in Ant-Man And The Wasp: Quantumaina as Woo tries to wrap up the Westview case.
Though Jimmy Woo referred to the missing witness as a "he" comic fans know Agatha Harkness has a lot of magical tricks up her sleeve. She could easily have appeared as other people to maintain her anonymity.
Eternals
It seems the Eternals movie will span a great deal of time, from the ancient past to the present day of the MCU. If that's the case, Agatha Harkness could appear in a couple of places. She could certainly appear in Salem in the 17th century if the movie visits there.
She could also theoretically appear much earlier if the Eternals establishes Atlantis. Agatha is connected to the fall of Atlantis through the magic of her mother and her coven. Her mother seems to share some characteristics with Zhered-Na, a powerful witch from Atlantis.
Black Panther: Wakanda Forever
Atlantis and Agatha Harkness could similarly factor into Black Panther: Wakanda Forever. If rumors are true and the movie does involve Namor, the prince of Atlantis, then it's likely fans will get a lot of Atlantean history.
If so, that could provide a way to show the fall of Atlantis and perhaps explain how Zhered-Na and her coven, including Agatha, found themselves in Salem. Right now it's a reach, but a potential avenue for the MCU to explore in the movie and in the future, including a potential storyline for the Worlds Of Wakanda Disney+ series.
Moon Knight
The Moon Knight series is likely to feature some mystical elements and as a consequence, perhaps Agatha Harkness. Agatha has been shown to be very interested in increasing her power, so she's probably spent a lot of the last few hundred years traveling the world to find it.
She may have gone to Egypt at some point, where in the comics at least Marc Spector encountered the Egyptian god Khonshu. Agatha would be very interested in the powers of Kohnshu, which depend heavily on the moon.
Secret Invasion
If the Secret Invasion series follows the plot of the Marvel Comics crossover, then it will be an all-hands-on-deck situation for the heroes of the MCU. A witch with Agatha's power would be very useful against an all-out Skrull invasion of the Earth.
Another possibility for the series is that Agatha appears, but is a Skrull in disguise. Skrulls have clearly infiltrated branches of government, including the FBI, seen at the end of WandaVision. The Skrulls might impersonate Agatha in their plot.
The Contessa's Dark Avengers
The Contessa is likely to make numerous appearances in the MCU in the near future, as Valentina Allegra de Fontaine is recruiting people for some mysterious purpose, and Agatha Harkness seems like the person that the Contessa likes.
Agatha is powerful and somewhat of a villain with a grudge against an Avenger. If The Contessa is putting together a team like the Dark Avengers or the Thunderbolts, Agatha Harkness would be a major get for the team.
Fantastic Four
Agatha Harkness' backstory has been changed for the MCU, but it remains a possibility that she could play a role in the upcoming Fantastic Four movie. Agatha first appeared in Fantastic Four #94 as the governess who looked after Franklin Richards, the son of Reed and Sue Richards.
One way this plays out is if the movie pays tribute to the FF's comic roots and begins in the 60s. The team could leave Franklin in the care of Agatha, at this point keeping a low profile as a nanny, but then they disappear in their fabled test flight, only reappearing in the present day.