Production got under way last month on Avengers 4, although Marvel Studios still hasn't released any official story details for the final Marvel Phase 3 movie. Since the currently-unknown Avengers 4 title is said to be a huge spoiler for next year's Avengers: Infinity War, it seems unlikely that we'll get too many details, at least until after Infinity War hits theaters. Today there is speculation that this movie may actually bring back Hayley Atwell as a possibly de-aged version of her character Peggy Carter, who passed away in Captain America: Civil War last year.
The actress posted a photo on her Hayley Atwell Instagram yesterday, which showed her with several dots on her face for motion-capture, while drinking from a large mug. She also added in her caption, "Don't question my process. It's very serious #setlifethuglife," and while she didn't specifically state what set she was on, many fans immediately thought that she might be in Atlanta filming the untitled Avengers sequel. Some think that the motion-capture dots may be used for the de-aging technology that Marvel Studios has used in the past, and is planning to use again in at least one upcoming movie.
The opening scenes of Marvel's Ant-Man featured a much younger Hank Pym, with Michael Douglas "de-aged" to appear a few decades younger than his real age. Marvel has also basically confirmed that it will use the same technology with Samuel L. Jackson's Nick Fury in Captain Marvel, which will be set in the 1990s and will de-age Nick Fury, showing him with two working eyes for the first time in MCU history. It is believed that this movie will show how in fact this iconic character lost his eye, which has never been revealed in the MCU up to this point.
If the actress is in fact on the Avengers 4 set, then the only we she literally could come back is through a flashback, since she passed away in last year's Captain America: Civil War. That movie also featured Peggy's love Steve Rogers dropping his shield as Captain America, and it has been confirmed that he's going to take on the Nomad look from the comics, although it hasn't been confirmed yet who will take over the Captain America mantle. In the comics, other characters like Bucky Barnes/The Winter Soldier (Sebastian Stan) and Sam Wilson/The Falcon (Anthony Mackie) have taken over as Captain America, but there still is no indication that either character will step in for Steve Rogers.
As for Hayley Atwell, it's possible that she will share more photos as her time on the set continues, and hopefully she might even tag the location on her next photo that will prove she's in Atlanta as Avengers 4 production continues. Even if Marvel does eventually release a full cast list for Avengers 4, it seems unlikely that Hayley Atwell's name will be on it, since there are often several cast members that are never officially announced by the studio, although sometimes the studio will reveal these cast members through tip sheets for the film's premiere screenings. While we wait for more on Avengers 4, take a look at Hayley Atwell's photo below, that may be taken from the Avengers 4 set.