The pair stated that most of what we were given in the story for both Infinity War and Endgame was roughly put together in the closing months of 2015. There were significant changes once the team began getting into their creative grooves. One significant change to the script came in the early death of Thanos, which was more of a complaint than a serious suggestion when the writers were having difficulty reconciling how to deal with the villain and his limitless power while possessing the infinity stones.
It turned out to be the perfect idea, and the idea progressed into the actual solution to their problem. “Killing him helps him as a character,” Markus said. “It cements the fact that he’s doing this for his stated purpose. He’s done! And that’s the most shattering thing for the Avengers.”
While the death of that character remained, the writers had difficulty deciding how to handle Captain America, as Chris Evans has been clear that he is finished with the role and the franchise after ten years of the same character. We saw the non-violent ending in which Steve Rogers lived out his dream with some version of Peggy Carter, while an earlier draft instead called for Thanos to return to the past and eliminate every avenger, dropping the head of 2012 Rogers at the feet of 2023 Rogers.
Would that have been too dark? Perhaps, but the plan was scrapped mainly because the concept was considered to be too confusing with the rest of the time travel mechanics.
The death of Tony Stark was also reconsidered various times; not so much in whether he would die, but more in how it would occur at the end. In the end, they chose for a silent death, where Stark was simply quiet, which seems like a great choice, as it is difficult to follow up the emotion and iconic, “I am Ironman.”
Professor Hulk and Thor were two other significant points of creative contention, mainly in that the writers had difficulty incorporating their characters in a way that felt tonally appropriate for each of the films. There were plans to have Bruce Banner reconcile his issues with the Hulk in Infinity War, but that felt out of place and was shelved. This may be why the original trailer for Infinity War showed us a charged Hulk in Wakanda alongside everyone else, whereas the theatrical release gave us Banner inside of the Hulk Buster armor from Age of Ultron. Thor, too, was reworked over and over again, simply because the writers felt that his original involvement “sucked rocks.”
Overall, fans are thankful that the film turned out as well as it did. It is no small task to bring together the sprawling storylines of characters after a decade of films spanning 23 titles in a way that does not falter.