Carol's new look after the time skip in Avengers: Endgame is inspired by the comics on two counts. Her short hair has been carried over from her modern design, as has the red sash she wears as a belt. Her new red costume with black shoulder-pads is lifted directly from Captain Mar-Vell's, Carol's predecessor. Additionally, it's subtle, but Carol showing concern for Rhodey and telling him to be safe is a reference to their recent romantic relationship in the comics.
When asked how 'Captain' was worthy to lift Mjolnir in Avengers: Endgame, according to Anthony Russo, he's always been worthy, and during that moment in Age of Ultron, he stopped himself from lifting the hammer fully out of deference to Thor: “In our heads he was able to wield it, and he didn't know that until that moment Ultron when he tried to pick it up. But Cap’s sense of character and his sense of humility, sort of out of deference to Thor's ego, you know Cap in that moment realizing he can move the hammer decides not to.”
At the beginning of Avengers: Endgame, Natasha gets a report from Okoye about an earthquake under the Atlantic Ocean. In the Marvel comics, the Atlantic is the home of the underwater kingdom of Atlantis, ruled by the Marvel hero Namor the Sub-Mariner. So this may be a nod to the future introduction of the character and the kingdom of Atlantis in the Marvel Cinematic Universe.
In the original Iron Man (2008), the soundtrack used a short excerpt from the classic rock song “Iron Man” by Black Sabbath. In Avengers: Endgame, Iron Man fulfills the line of the song, “when he traveled time, for the future of mankind.”
In Avengers: Endgame, Thor was supposed to confront his 2012 self, but the idea was ultimately scrapped as revealed by directors Joe and Anthony Russo. While it was Captain America (Chris Evans) who had a direct entanglement with himself from the events of The Avengers' Battle of New York from 2012 and Nebula (Karen Gillan) with her evil counterpart from 2014's Guardians of the Galaxy, apparently, Thor was supposed to have his own similar moment in Endgame. Speaking with MTV's podcast, the Russos said that the initial plan was to have a Thor vs. Thor confrontation in Asgard. However, they felt like "it overly complicated [the plot]," adding that they also liked the “Cap vs Cap better” so they gave up the bit. Instead, they gave Thor a different experience when he went back in time - another chance to talk to his mom and properly say goodbye to her commenting: “I think there, we also deferred to the storyline between Thor and his mother. [It] was so resonant, that we really wanted to run... That was really more a part of Thor's journey and repair than confronting his former self. So that's really what happened there, why we went with that.”
Thanos' infinity gauntlet was worn on the left hand. When Tony Stark creates a new gauntlet in Avengers: Endgame, it was worn on the right hand. Swapping hands symbolizes the reversal of Thanos' snap, for which the new gauntlet was created.
At Tony Stark's funeral in Avengers: Endgame, a teenage boy is seen standing behind Scarlet Witch and Bucky. This is Harley Keener (Ty Simpkins), the boy from Iron Man 3 (2013) who let Tony use his shed when he needed to repair his Iron Man suit.
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In Avengers Assemble (2012), Captain America tells Iron Man, "You're not the guy to make the sacrifice play," and Iron Man tells Captain America, "Everything special about you came out of a bottle." In Avengers: Endgame, each proves the other wrong. Iron Man sacrifices himself to defeat Thanos, while Captain America proves himself worthy by being able to wield Thor's hammer, Mjolnir.
29Avengers: Age of Ultron
In the Avengers: Endgame’s final battle, Thanos breaks Captain America's shield in half, fulfilling the prophecy which was shown to Tony in Avengers: Age of Ultron (2015). Also shown in the vision is Hulk injured and unable to fight, similar to Avengers: Infinity War (2018), Black Widow dead in the same position as she dies in “Endgame”, and Thor appearing to be dead in the same position as when Thanos wounds him with Stormbreaker in the final battle.
Avengers: Endgame is the first Marvel film to not have a post-credits scene. Many theatergoers would stay behind in anticipation of the continued tradition, but instead, there is a blank screen and a brief clanging sound. The sound heard is Tony Stark's hammer from the first film (Iron Man (2008)). This is an indicator from Marvel that the events are final and marks the official end of the decade-long Infinity Saga.