Avengers Infinity War & Endgame co-writers Christopher Markus & Stephen McFeely started outlining both movies back in 2015, and revealed in an interview with Yahoo Entertainment there were some major changes that happened along the way.
First of all, whatever tweaks they made to Infinity War had ripple effects for Endgame, so a quick change here had a big effect there. Markus & McFeely revealed the opening scene of Endgame where Hawkeye watches his family disappear into dust was originally written to be the final scene of Infinity War.
The writers also revealed they planned to unveil "Smart Hulk," the blending of the giant green beast and his alter ego Bruce Banner in Infinity War, but unfortunately after shooting most of Smart Hulk's scenes in Infinity War, they realized there was no clear explanation for how he came about.
Markus & McFeely also talked about how they drastically changed their original plan for time travel. Originally almost everyone traveled to different places than what we see in Endgame, but ultimately settled on what we see on the big screen to offer the most closure to the characters and story lines that need it.
They also toyed with the idea of bringing back Vision, at the beginning of Enbdgame Captain America would be struggling with his relationship with live-in girlfriend Sharon Carter and Tony Stark going to the “ethereal world” and having a conversation with the adolescent version of his daughter Morgan, played by Katherine Langford from Netflix's 13 Reasons Why.
Watch the interview below for all the behind the scenes secrets!