Marvel Studio head Kevin Feige announced at San Diego Comic-Con on Saturday that the upcoming Captain Marvel movie starring Brie Larson as the fighter-pilot-turned-superhero will take place in the 1990s. He also revealed that Captain Marvel will be battling a group of aliens called the Skrulls. The Skrulls are shape-shifting aliens and can pretend to be anyone else, so the introduction of these characters could upend the entire Marvel Cinematic Universe. Let me explain.
The Skrulls are usually green but can take the form of any animate or inanimate objects (including humans). Because of their ability to imitate other forms, they often serve as spies.
Skrulls traditionally have appeared in the Fantastic Four universe. Considering that 20th Century Fox, not Marvel Studios, own the rights to the Fantastic Four franchise, Marvel head Kevin Feige presumably had to do some negotiating to get his hands on these characters.
Whatever happened behind the scenes, this means Marvel can now dive into the comic lore surrounding certain characters, like in the Kree-Skrull War. (As a reminder, Ronan from Guardians of the Galaxy was Kree.)
It also means that because the Skrulls can masquerade as anybody, any character in the Marvel universe could now be revealed to be a Skrull impostor. Some on Twitter are already theorizing that some Avengers in Avengers: Infinity War could reveal him or herself to be a Skrull.