What Captain America 3 was originally going to be about, according to Kevin Feige
Before Civil War had been picked as the source material for Captain America: Civil War, a different plan was in the works, as Kevin Feige has revealed to io9. The mastermind behind the Marvel Cinematic Universe initially wanted to follow on directly from The Winter Solider.
“We developed Captain America 3 knowing we wanted to continue the Bucky story. Is Bucky going to get his mind back? What is Bucky struggling with, after the tag scene on Winter Soldier, and how does Steve’s desire to save Bucky bring him into conflict with something else, thinking about how do the sins of his past sort of affect him?
" And [writers] Chris [Markus] and Steve [McFeely] came up with a number of cool plots that could’ve worked, but none of them were feeling worthy of a follow up to Winter Soldier.”
Stephen McFeely reiterated these statements, and expanded on them by stating that “Even before Civil War came down the pike, we were already working on the Bucky part of it all.”
So it sounds like, as opposed to an adaptation, early talks had the film going in an original direction. We can ponder that while we wait for Civil War to arrive.
Source: Screenrant