Despite it being perhaps the most discussed and anticipated film in the history of cinema, fans still really don’t know what to expect from Avengers: Endgame. Even after the theatrical release of Captain Marvel - which loosely serves as a lead-in to Endgame - most MCU Phase 4 theories are still speculative at best.
To be sure, fans should have a clearer picture of the Marvel Cinematic Universe moving forward after watching Captain Marvel. The big screen debut of Carol Danvers greatly expands upon the cosmic branch of the MCU and drops some tantalizing hints about things to come, all of which have resulted in the internet being deluged with Captain Marvel-based fan theories for Avengers: Endgame and beyond.
Many fans who saw Captain Marvel immediately assumed the film’s big twist - that the Skrulls are not an evil race of alien shapeshifters but instead a neutral race of alien shapeshifters - nixed any chances of the infamous Secret Invasion storyline happening in the MCU. Redditor /u/Jellymenn, however, feels differently.
Noting the time difference between Captain Marvel and the modern Marvel Cinematic Universe, Jellymenn notes how the unnamed daughter of Talos experiences some rather trying events throughout the course of the film - which could lead to her growing up as a human-hating villain of sorts. Specifically, they theorize Talos’s daughter will grow up to be Veranke, the Skrull queen responsible for the titular infiltration of Earth in the comics.
The first post-credits scene of Captain Marvel features Carol Danvers answering Nick Fury’s beacon, appearing directly behind Black Widow inside Avengers HQ. There aren't any alarms going off in the scene, so either Danvers broke into the facility or - as Redditor /u/CleverD3vil theorizes - someone with an access card accompanied her.
The theory states that Captain Marvel will rescue Tony Stark - and presumably Nebula - from their space-marooning and take them to Earth. Therefore, Carol doesn’t need to break into the Avengers’ facility - Tony will just open the door.
At the end of Captain America: The First Avenger, Howard Stark fishes the Tesseract out of the ocean, but Captain Marvel reveals it later ends up in the hands of Mar-Vell, a renegade Kree disguised as a human scientist.
A theory by Redditor /u/ANarn214 posits the answer to how the Space Stone-containing cube changed ownership from Stark to Mar-Vell is rather simple: the two super-geniuses were working together on Tesseract research. Mar-Vell did work for a branch of S.H.I.E.L.D., so it’s not much of a stretch to imagine the scientists collaborated at some point.
Such a reveal would tie the origins of Tony Stark and Carol Danvers together in a meaningful way, further solidifying Captain Marvel’s presence in the fabric of the MCU.
The last post-credits scene of Captain Marvel features Goose the Flerken regurgitating the Tesseract on Nick Fury’s desk - which means S.H.I.E.L.D. has been in possession of an Infinity Stone for more than a decade by the time Avengers rolls around. But what were they doing with it?
According to a theory by Redditor /u/Britator, S.H.I.E.L.D. has been experimenting with the Tesseract in an attempt to recreate the accident that granted Carol Danvers her powers - to make more Captain Marvels! There’s a boatload of possible implications if this information were ever to become public. As Britator notes:
[S.H.I.E.L.D.] weren't trying to make weapons - Hydra had figured that out fairly easily, and Captain America had captured dozens of them. They weren't trying to make an energy source either - Tony Stark and Ivan Vanko both successfully and independently miniaturized the arc reactor technology. So either [S.H.I.E.L.D.] scientists are all idiots, or they had another goal in mind.