When it comes to the Marvel-verse, there are many standard fantasy tropes the series has attempted to avoid. And the most prominent comic staple that the movies have sidestepped would have to be time travel. It's an natural choice. Time travel is hard to get right, and it is easy to create plot holes. But when Avengers: Endgame came along, the Russo brothers decided it was time to tackle it. In doing so, they drew from an unexpected source, with writers Christopher Markus and Stephen McFeely admitting Harry Potter inspired time travel in Avengers: Endgame.
Avengers: Endgame's time travel was technically used to go back and gather all the different Infinity Stones from the various places they were first seen around the MCU. Once obtained, the remaining Avengers planned to put them all on a makeshift gauntlet and snap everyone back into existence that Thanos wiped out. So simple, yet so complicated.
As a plot device, this allowed the film to revisit three previous films that built up to Endgame and visit with various characters from the Marvel-verse. In particular, it meant running into The Ancient One, who was the bearer of the Time Stone before Dr. Strange. She is the one who explains the rules of time travel to a slightly bewildered Hulk.
But Hulk wasn't the only one who was befuddled. Many fans watching were confused about how time travel worked by the time she was done explaining. That's because most viewers are used to time travel done Back To The Future style.
Speaking to ComicBook.com, McFeely explained:
We went back and watched both Back to the Future and Back to the Future 2 specifically. Everyone thinks that's how time travel works because that's a great movie, maybe the best of its subject — but if we were to [follow those rules], to do something in the past it's gonna screw up your future, we're going to do that six times...
Since the rules of time travel from Back To The Future didn't work for the film, they had to look elsewhere. Thankfully, Harry Potter and J.K. Rowling were there to provide.
McFeely felt the sort of time travel seen in Prisoner of Azkaban would suit their needs.
I do love that third Harry Potter movie, where [we see] a stone break a vase. You don't know why, but the scene's fine, and it doesn't take you out of it," McFeely said. "Then when you come back around [during the time travel], and you realize that THEY had thrown it at THEMSELVES, I do love that.
That's all well and good, but it also explains why so many fans were upset by Endgame's time travel rules. After all, Rowling herself admitted she had no idea what she was doing when she decided to create the Time Turner, and realized in horror all the plot holes it was creating. It's why the device disappears after the third installment never to be seen again.
Thankfully, Captain America also used up all the time travel at the end of Endgame to go live out his life with Peggy Carter. It, too, can disappear from the MCU, never to be used again.