One of those subplots was described in Markus and McFeely’s notes as simply “Steve Ages: Some dimensional horseplay could result in Steve Rogers aging to his actual hundred-year age.” However, it was a whole other abandoned Captain America thread that really grabbed McFeely’s attention during the interview:

“Holy crap, Chris, it says, ‘Back to the Future: Steve somehow lands in 1945, realizes that he needs an Infinity Stone in order to return to the present. Fortunately, Howard Stark has fished it out of the North Atlantic.’ [Laughs] So yeah, it really all stems from the manifesto.”

It seems this story thread may have been an early prototype for a subplot that actually did make its way into “Endgame.” The “Back to the Future”-y sequence in question sees Steve and Tony Stark (Robert Downey Jr.) make a quick detour to 1970, where Tony encounters the middle-aged version of his father Howard played by John Slattery. One can understand why Markus and McFeely originally had their eyes on sending Steve back to 1945, though. That would’ve seen him instead crossing paths with Dominic Cooper’s Howard Stark, the younger iteration of the character the two writers introduced in “Captain America: The First Avenger” before spinning him off into the “Agent Carter” series (which they co-created).

Presumably, the deserted 1945 subplot would’ve also seen Steve getting a glimpse at Peggy Carter (Hayley Atwell), thus setting up their eventual happy ending. Ultimately, by going with the ’70s thread, Markus and McFeely were able to kill two birds with one stone, laying the foundation for Peggy and Steve’s reunion while also providing closure to Tony’s relationship with his late father. It’s that type of economic storytelling that prevents “Endgame” from collapsing under the sheer weight of its many, many moving parts.