It's no secret that once the Skrull threat to Earth is eradicated, Marvel Studios' Captain Marvel will have set up Brie Larson's superpowered Carol Danvers to enter Avengers: Endgame, presumably as an equalizer enlisted to take down Thanos.
As for how she joins the fray and attempts to save a galaxy in disarray after half its life was snuffed out, well, that remains a mystery. Numerous theories have been floating around the Internet, of course, but thanks to Larson's co-star, Samuel L. Jackson, we may have to pour cold water on some of the more popular hypotheses.
The Old Theories...
When it was announced Danvers' origin film would be set in the '90s, one of the first questions fans had was how this could be considering Nick Fury didn't mention her in 2012's The Avengers. As Earth's first true superhero, you'd think people would discuss her as Iron Man, Thor and the rest arrived on the scene. She seems to have been erased from our planet's history, with the how and why being the source of several Endgame fan theories.
Many assume that at the end of Captain Marvel, she'll be trapped or willingly placed in suspended animation, maybe recovering from the aftereffects of the Skrull war. Fury's pager has been thought to be a beacon that would break her out of her slumber, signaling her arrival into Endgame where she'll attempt to learn why her former partner turned to dust at the end of Infinity War.
Come Ant-Man and the Wasp, though, these theories took a different direction when we got confirmation the Quantum Realm (aka, the microverse) could allow users to traverse both space and time. Speculation now shifted to the idea that someone from the present -- possibly Paul Rudd's Scott Lang -- would go through time recruiting heroes, including Carol, to fix Thanos' mechanisms. Or, Carol may have stumbled upon the Quantum Realm herself, where she'll save Scott (who was trapped in the microverse at the end of his sequel) and bring them both to a post-snap Earth.
However, according to Jackson's words (which we're taking with a slight grain of salt), may want to scrub both these theories. According to Nick Fury, Captain Marvel has the power of time travel in her arsenal.
"[Captain Marvel is] pretty much the strongest character -- in terms of someone with powers are able to do things -- in the Marvel Universe," Jackson revealed as part of his Captain Marvel interview junket. "We know that we need something that's as powerful as Thanos. And at some point, we'll find out how powerful she is and all the things that she's capable of. She's one of the few people in the Marvel Universe that can time travel, so..."
Now, this could be an error on his part, but Jackson knows his comics. While she hasn't displayed full-blown time travel abilities in Marvel Comics' continuity, Marvel Studios may have tweaked her powers.
As Binary, one of Carol's other superhero identities, she could tap the energy of a white hole, allowing manipulation of stellar energies, the electromagnetic spectrum and gravity. It's entirely possible her movie counterpart will share these powers, adjusting them in such a way that she can create rifts to traverse the space-time continuum. With time travel at her disposal, it's possible Carol may end up piercing the time barrier a la The Flash, and moving decades ahead to come to Fury's aid.
Time travel is a big asset for any hero to have. Admittedly, it could make Captain Marvel a tad overpowered, though considering the Time Stone is in Thanos' possession, it might just be enough to level the playing field when the Avengers regroup and attack him once again. If she does turn out to have this ability, Carol could be the greatest deus ex machina ever, ready to go back in time and change the course of history by preventing Thanos' genocide before it's even begun... possibly rewriting MCU history in the process.
Directed by Anna Boden and Ryan Fleck from a script they wrote with Liz Flahive, Carly Mensch, Meg LeFauve, Nicole Perlman and Geneva Robertson-Dworet, Captain Marvel stars Brie Larson as Carol Danvers, Samuel L. Jackson as Nick Fury, Jude Law as the commander of Starforce, Clark Gregg as Phil Coulson, Lee Pace as Ronan the Accuser, Djimon Hounsou as Korath the Pursuer, Gemma Chan as Minn-Erva, Ben Mendelsohn as Talos, Lashana Lynch as Maria Rambeau, Algenis Perez Soto as Att-Lass, McKenna Grace as a young Carol Danvers and Annette Bening in an undisclosed role. The film arrives on March 8.