When the credits roll on Avengers 4, it won’t take long for fans to stop talking about whether their favorite fan theories came true - and to begin speculating what’s next for the Marvel Cinematic Universe. The MCU plans to stick around for years to come, and that means the franchise will have to move on from Thanos and the Infinity Stones to a new overarching conflict. Many fans suspect that conflict will draw heavily from the "Secret Invasion" storyline that took place across Marvel's comics in 2008.
The crossover event centered around the shapeshifting race of aliens known as the Skrulls. Long-time Marvel villains dating back to the early '60s, the Skrulls pulled off their most insidious feat in "Secret Invasion:" embedding Skrull sleeper agents throughout the ranks of Earth’s Mightiest Heroes to take over the planet. Fans already know that the Skrulls will make their debut in 2019’s Captain Marvel, set in the '90s. This means they’ve been active in the MCU for decades - plenty of time for them to spirit away and replace any number of MCU characters.
Throughout the first three phases of the MCU, some of our favorite characters may have been Skrulls all along. The truth will eventually be revealed, but for now, it’s up to fans to wildly speculate and theorize about which of their favorite heroes and villains might be shapeshifting alien imposters.
Whether hunting down the Hulk or drafting the Sokovia Accords, General Thaddeus “Thunderbolt” Ross (William Hurt) has been one of the most persistent opponents of superheroes in the Marvel Cinematic Universe. Redditor /u/meme_abstinent theorizes that Ross’s anti-superhuman agenda is part of a Skrull plan paving the way for an eventual invasion.
Getting the Avengers out of the way certainly suits the Skrulls’ purposes, and Ross is the exact sort of government influence the shapeshifters would benefit the most from replacing. Suddenly, his intense hatred of the Hulk looks more like strategy than obsession, and that makes Ross an infinitely more interesting character.
Appears In: The Incredible Hulk, Hulk, Marvel Universe
From the British version of The Office to Sherlock, Martin Freeman specializes in roles that audiences naturally trust and empathize with. It would be a perfect subversion of his reputation if the MCU revealed his character Everett Ross as a Skrull.
It’s not all that far-fetched, either, according to a theory put forth by Redditor /u/Knightboat17:
In Black Panther, its mentioned Ross was previously in the Air Force, I estimate his age to be late 40s same as Martin Freeman, so he would be in his 20s during the 1990's where Captain Marvel is set, I can see him appearing in it as a member of Carol's unit and a maybe even a close friend/love interest.
Furthermore, during the film, he may go missing, and we discover through a post-credit scene that he is one of the characters the Skrull have replicated to start their invasion. It would be useful for a high-ranking member of the CIA to be one and now that he is one of the few outsiders to have visited Wakanda.
Some fans are worried that the Skrull revelations will reboot or ruin some of their favorite characters, effectively rewriting several films' worth of character development as nothing more than an alien charade. As Redditor /u/UmbrusNightshade argues, Marvel Studios could avoid that by choosing a character that has been around for a while without receiving much development: Maria Hill (Cobie Smulders).
As Nick Fury’s right hand, Hill has access to all of Earth’s secrets, which would make her a useful spy for the Skrulls. And as someone who's been around since the beginning, Hill could have feasibly been manipulating Fury, S.H.I.E.L.D., and the Avengers for her alien allies. Such a revelation could have deadly implications.
Appears In: The Avengers: Age of Ultron, Captain America: The Winter Soldier, The Avengers, Marvel Universe
If Marvel wants to wring the most emotion from a Skrull reveal, the shapeshifting alien would have to be impersonating a character that's been around since the beginning. And there’s no better fit than Tony Stark’s best friend James Rhodes (Don Cheadle).
As the YouTube channel Nando v Movies outlines, there's a lot of evidence that makes War Machine the perfect Skrull candidate. For one, Rhodes has never appeared in a scene by himself. This means there's no continuity question as to why a Skrull would impersonate Rhodes with no one around to see. He’s also never had an onscreen love interest, which would negate some of the creepier implications of shapeshifting double-agents.
War Machine also survived a horrific fall in Civil War that could have killed a regular human. The alien anatomy of the Skrull could explain to how Rhodes survived. Skrulls can shift their internal organs around to survive catastrophic injuries, and that may explain why Rhodes is already back in action for Infinity War. As a survivor of Thanos's Snap, he’s now perfectly positioned to signal the beginning of the "Secret Invasion."
Appears In: The Invincible Iron Man, Iron Man, Iron Man 3, Iron Man 2, Marvel Universe
Powers & Abilities: Flight, Superhuman strength