Since 2008's Iron Man, the Marvel Cinematic Universe has been angling towards preventative measures for a universe-ending threat, ergo why Samuel L. Jackson's Nick Fury made it clear the world needed the Avengers initiative. A decade later, Avengers: Infinity War brought this fear to life with Thanos snapping half the universe out of existence.
With Avengers: Endgame bookending the journey of Earth's Mightiest Heroes, the Guardians of the Galaxy and the Mad Titan, we're left wondering what will be the big hook when Phase Four kicks off in the MCU. Given what we've seen from Captain Marvel so far, and the narrative it's pushing about Skrull infiltration and conquest, this future can only lie in an arc influenced by Marvel's 2008 event Secret Invasion.
Fans have been speculating since summer of 2017 that Secret Invasion would be on the docket, as the Skrulls had been confirmed as the main villains of Captain Marvel. Many felt this would be the kind of clandestine event that could flip the MCU on its head, building towards a threat disguised internally as opposed to something from the stars, like Thanos. Marvel Studios itself has done little to pour cold water on this theory, with one of Captain Marvel’s executive producers even confirming what they're doing with the Skrulls actually plants these seeds.
"We talked about Secret Invasion. I wouldn’t go into this movie saying, ‘Which one of these superheroes is a Skrull?’ because Carol is more or less the first superhero to show up on Earth," said Jonathan Schwartz. "But are we planting seeds for future movies? Always." This game of “Who’s the Skrull?” played with MCU heroes may not be applicable as Phase Three winds down with Endgame and Thanos, but it's the perfect draw for what comes next.
It's unlikely that shapeshifting villains like the Skrulls will be a one-and-done deal, used only in Captain Marvel and then axed. After knowing what they can do, who they can impersonate and how they can infiltrate positions of power, an Endgame post-credits scene teasing one as high-ranking member of government like Thaddeus Ross (who has nuclear capabilities at his fingertips) or a fallen hero would be the biggest tease for what's to come.