Despite the Infinity Stones being clearly established as the key intergalactic MacGuffins across the first three phases of the Marvel Cinematic Universe, the Soul Stone remained conspicuous by its absence. The Space Stone was seemingly everywhere throughout Phase One, the Mind Stone led to the creation of Vision in Age of Ultron, and the Reality, Power and Time Stones were the driving forces behind the plots of Thor: The Dark World, Guardians of the Galaxy and Doctor Strange respectively.
We didn’t get our first glimpse of the Soul Stone until Avengers: Infinity War, when it was revealed that the Red Skull had been guarding it on Vormir since the end of Captain America: The First Avenger. Obtaining it requires the ultimate sacrifice, which ultimately led to the death of both Gamora and Black Widow as both Thanos and Earth’s Mightiest Heroes sought to bring their own kind of balance to the universe.
While fans love the idea of how the Red Skull would have reacted when his arch-nemesis shows up out of the blue on Vormir almost eighty years later and simply hands it back to him during his adventures through time hinted at during Avengers: Endgame‘s final moments, in a recent watchalong for the movie, the Russo brothers were responding to fan questions and dropped the bombshell that during the early stages of development, Cap himself was considered to be the Soul Stone.
— Russo Brothers (@Russo_Brothers) April 28, 2020
While that would have no doubt been a shocking reveal that would have led to the Avengers being forced to make some big decisions, you can understand why the Russos quickly dropped the idea. There seems to be little way for it to fit organically into the story they were trying to tell in Avengers: Endgame and it would have ended up being a twist just for the sake of it.