Joss Whedon intended to introduce Captain Marvel years ago, in Avengers: Age of Ultron - but Kevin Feige had other ideas. Starring Brie Larson as the titular hero, Captain Marvel is the MCU's first female-led solo superhero film. So far, it looks set to be another blockbuster hit; IMAX presales are tracking ahead of Wonder Woman or Thor: Ragnarok.

Fans really began asking for a female superhero movie back in 2012, in the aftermath of The Avengers. Marvel took notice, and a number of names were tossed out in interviews at the time; Black Widow, Peggy Carter, Pepper Potts - and Captain Marvel. Kevin Feige favored doing an origin story, and so attention shifted on to Carol Danvers. In October 2014, Marvel finally confirmed that Captain Marvel would be part of the Phase 3 slate.

Related: How & Why Marvel's Phase 3 Slate Changed So Much

The only question was just how Marvel should introduce her, and rumors began to circulate that Captain Marvel would actually make her debut in Avengers: Age of Ultron. Surprisingly, Kevin Feige admitted these rumors were actually based on early drafts of the script; as he explained in an interview with Birth.Movies.Death, Joss Whedon had been keen to introduce Carol Danvers.

"[Captain Marvel] was in a draft. But to me, it would have done that character a disservice, to meet her fully formed, in a costume and part of the Avengers already when 99% of the audience would go, 'Who is that?' It’s just not the way we’ve done it before... The way we reveal Scarlet Witch [in costume] at the end of the movie? Those were Captain Marvel plate shots. Joss said, 'We’ll cast her later!' And I said, 'Yeah Joss, we’ll cast her later.' [Whispers to an invisible associate who isn't Joss] 'We’re not putting her in there!'

Finally Joss was like 'Let’s use those plates to let Scarlet Witch fly into frame, give her a big entrance?' And that makes sense - she’s come to their side, and she deserves the cool intro, which will feed into another movie we start shooting in a few weeks."

Aaron Taylor-Johnson as Quicksilver and Elizabeth Olsen as Scarlet Witch in Avengers: Age of Ultron

In Feige's view, Captain Marvel was a title character who deserved better than to be randomly tossed in at the end of a movie. He compared Whedon's idea with the introduction of Thanos; the Mad Titan was introduced in such a way as to clearly be a villain, setting up a major storyline. In contrast, this cameo would have felt utterly random, with viewers not having a clue who this new character was. So the FX plates were ultimately used to introduce Scarlet Witch instead. That fitted perfectly with Wanda's character arc; by the end of the film she'd chosen to step out and battle Ultron alongside the Avengers, proving herself a worthy member of the team. What's more, the flight scene subtly expanded Scarlet Witch's powerset, setting her up for Captain America: Civil War and even Avengers: Infinity War.

Curiously, though, one aspect of Whedon's idea has remained. He clearly imagined a scenario where Carol Danvers was already an established superhero in the MCU, albeit one who'd never been seen or acknowledged by the films before. In just the same way, Captain Marvel is set in 1995, essentially serving as a backdoor prequel to the entire MCU. That means Carol Danvers will have been serving as a superhero for decades before she teams up with the Avengers in battle against Thanos in Avengers: Endgame.

It's fascinating to imagine how differently the MCU would have played out had Whedon gotten his way. Carol Danvers would have been part of both Captain America: Civil War, where she'd presumably have picked a side. Perhaps she'd have chosen Team Cap, with her phenomenal power-levels matched against Vision's. Whatever decision she'd have made, though, Captain Marvel would surely have been on hand to battle Thanos when his forces invaded Earth in Avengers: Infinity War - but it's impossible to say how things would have turned out.

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