Two recent casting calls confirmed that either Avengers: Infinity War or Avengers 4 will travel back in time to the 1960s.
The Central Casting Georgia casting calls are for female extras to play secretaries and engineers from the 1960s in a project titled Mary Lou. Shooting will take place on October 10 and 11 for the scenes with secretaries, and the scenes with engineers will shoot on October 12, 13, and 16. Check out the specifics:
Secretaries: Looking for women who appear to be Caucasian, 20s-50s, to portray 1960s secretaries/clerks. Hair must be period-appropriate: no longer than shoulder length, no dye, no modern colors or cuts. Must be dress size 0-8.
Female Engineers: Looking for women who appear to be Caucasian, 20s-50s, to portray 1960s engineers in this VERY small scene. Hair must be period-appropriate: short hair, no longer than shoulder length, no dye, no modern colors or cuts. Must be dress size 0-8.
Mary Lou is the working title for Avengers: Infinity War, and the Russo Brothers have wrapped production for that movie. But the working title for Avengers 4 is Mary Lou 2. It's not quite clear whether the flashback scenes will be part of the May 4, 2018 or the May 3, 2019 release.
Going back to the 1960s could mean that Peggy Carter, Howard Stark, Hank Pym, or even all three of them will be involved in either upcoming Earth's Mightiest Heroes installment. But it’s all speculation at this point, so we’ll need to wait until we have some concrete evidence.
These movies are the culmination of everything that’s taken place in the Marvel Cinematic Universe, so it feels safe to assume that the flashback scenes would have some significance in terms of storytelling. We don’t think there would be any room for random scenes in either movie.