All spoilers on this page are left unmarked. You Have Been Warned!
Unsurprisingly, the road to the movie has seen a huge amount of theories about what'll happen, with people analysing pretty much everything in the name of clues.
- The identity of Peter Dinklage's character naturally created quite a bit of speculation as soon as he was confirmed to be a part of the cast, with the two most common guesses being that he was M.O.D.O.K. or Pip the Troll, and others got a bit more creative by suggesting that he could be a different character altogether (specifically, Eitri, an obscure character that an alleged leaker claimed was the creator of the MCU's Infinity Gauntlet). Since a later image of Dinklage showed that he dyed his hair red right before filming began, speculation leaned more toward Pip and Eitri. In the end, he played Eitri, who was in fact the creator of the Infinity Gauntlet.
- The casting call given to two young twin boys, combined with the reports of a kiss and "an exciting storyline" between Wanda and Vision, has lead many to suspecting the introduction of Wiccan and Speed into the MCU. They do not appear — at least not in the first movie — and both Wanda and Vision are among Thanos's victims.
- Some people think the "get this man a shield" line could be directed towards Bucky rather than Steve, since many think the most likely death in this film is Steve's. It helps that the MCU has been gradually showing Bucky's improved skill with Steve's shield over several films. Turns out inverted; Bucky dies to Thanos's manipulations while Steve survives.
- A LEGO set of the Sanctum Sanctorum included a picture of Daredevil in a newspaper, raising rumors that he might actually be in the film given their previous spoiling of Giant-Man in Civil War. Turns out he isn't.
- The Soul Stone is still unrevealed going into the film. The most popular guess is that it's in T'Challa's ring, though this guess appears to have been jossed with the release of Black Panther. It's actually on a random planet, guarded by the Red Skull.
- Many fans began to speculate that Loki would be one of the first casualties of the movie. This is due to him lacking any appearances in promotional materials beyond the trailers. Those fans were completely right.
- Loki's early demise and status as a fan favorite gave rise to countless theories about how he managed to fake one more death, despite Thanos' "No more resurrections" line. So far, of course, he remains dead.
- The fact that "fantastic" was written in all-caps in the announcement tweet for the release date change has people theorizing that the Fantastic Four will be introduced or otherwise mentioned in the film. They are not.
- The complete lack of Hawkeye and Ant-Man in the advertising, despite both actors having long since confirmed that they're in the film, has a lot of fans speculating that they play some incredibly important role that showing any part of would be a huge spoiler. An especially popular theory is that Hawkeye has been sent to find Captain Marvel. A line mentions both are on "house arrest", and what role Clint and Scott will have, audiences won't find out until upcoming movies.
- After the movie was released, some comic fans began speculating that the child Gamora with Thanos in the ending is in fact his Mistress Death, otherwise Adapted Out, taking her form.
- Given that he's one of the only people who survived, some have suggested that M'Baku will take the throne of Wakanda in light of T'Challa's disintegration, or that Shuri (assuming she didn't die too) will become the new Black Panther instead. Or both.
- Banner's inability to transform into the Hulk (and suspicious level of knowledge about the Infinity Stones) has led some viewers to theorize that the Banner we see on Earth is actually Loki disguised as him.
- The fact that Jane Foster was one of the several characters whose fates were "too spoilery" for the Russos to expound upon has opened up a fair amount of speculation, seeing as basically everyone assumed the character had been permanently retired from the franchise.
- The fact that Dr. Strange and Loki both offer up Infinity Stones in order to save a comrade (the Tesseract for Thor and the Time Stone for Tony, respectively) in almost exactly the same manner, complete with very similar lines afterwards, has become a flashpoint of speculation, since both are explicitly sorcerers and the scenes are far too similar to be coincidental. Some people have even suggested that Loki and Dr. Strange were actually in contact with one another during the film, and have taken it as an indication of various magical shenanigans to be explained in the sequel.
- Downplayed, but some post-release theories put much more stake in the line "You should have gone for the head..." then they likely would otherwise, as some fans seem to believe that Thanos' head serves as a Zelda boss-esque weak spot, which would have one-hit killed Thanos if a character succeeded in hitting him there, rather than assuming the line just serves as a cheeky Pre Ass Kicking One Liner on Thanos' part, as was probably intended.