Captain Marvel almost made her Marvel Cinematic Universe debut in Joss Whedon's Avengers: Age of Ultron, and a new alternate scene shows the early version of her cameo. Brie Larson only brought Carol Danvers to the big screen at the beginning of this year in Captain Marvel and Avengers: Endgame, but that was not always set to be how Marvel Studios brought her to the screen.
During the development of Avengers: Age of Ultron, reports started to surface that casting was underway to try and find someone to play Captain Marvel in the movie. This came after reports that the screenplay for the film included mentions of Ms. Marvel and other heroes joining the Avengers at the very end. There was plenty of speculation over how she could be included in the film, but once it was released, Captain Marvel was, in fact, not part of the story. Some details of what could've been have surfaced in the years since Age of Ultron's debut, but Marvel Studios has refrained from showing fans this possibility. Thanks to the release of the Infinity Saga box set though, that has changed.
One of the never-before-seen deleted scenes featured in the box set shows fans what could've been. The alternate scene begins just like Avengers: Age of Ultron's actual ending, with Captain America and Black Widow walking into the hanger at Avengers HQ to see their new lineup. Falcon drops in as usual, but then Vision and Scarlet Witch are shown in the same shot before transitioning to War Machine. Then, the scene cuts and the camera pans up to reveal a blonde-haired woman standing heroically. Just so anyone watching the deleted scene understands, the scene then rewinds and points an arrow at her as "Captain Marvel Stand-In" is added above.
So why was this Captain Marvel cameo scrapped from Avengers: Age of Ultron? The idea for Carol's introduction came from director Joss Whedon, who wanted to have this surprise close out the film and cast an actress at a later date. But, Marvel Studios president Kevin Feige has been pretty open about the fact that he wasn't a fan of the idea. He's said that they didn't want to introduce a character as important as Captain Marvel in this capacity. This would've amounted to only a few seconds of screentime for a brand new actress to make her mark instead of giving someone an entire movie of her own and goes against how Marvel has traditionally introduced new heroes.
But, the implications of this scene are still staggering to consider if Marvel did include this ending. The MCU would've introduced Captain Marvel four years earlier and in a very different way. Instead of a solo film showing her origin, audiences would first meet Carol when she's already become a member of the "New Avengers." She would likely then have gone on to be featured in Captain America: Civil War. As for her solo film, it's unlikely it would've come any sooner since Marvel already announced Captain Marvel prior to Avengers: Age of Ultron's release, but it may not have been a 1990s origin film if she was featured multiple times before. Phase 3 of the MCU and beyond would clearly look very different in Avengers: Age of Ultron completed this scene.