The latest trailer for Avengers: Endgame, released months after set photos leaked online, seemingly confirmed that Earth's Mightiest Heroes return to the Battle of New York from 2012's The Avengers. Of course, that was the climactic fight, in which Nick Fury assembled the team to stop Loki and the Chitauri invasion of Earth. Of course, audiences learned in a post-credits scene that those antagonists were merely Thanos' lackeys.
This new insight for the Marvel Studios sequel may provide deeper insight into why the surviving Avengers travel back in time to that moment. And it could change everything about the Marvel Cinematic Universe.
That battle was Thanos' first indirect interaction with Earth's heroes. As the Other told him, to challenge Earth's warriors would be to court death itself, which is why the Mad Titan took note of Tony Stark, who led the charge against his alien army. But by traveling through time, presumably by way of the the Quantum Realm, the Avengers may be heading there not to fire a warning shot at Thanos, but instead to claim two Infinity Stones.
We know Loki possessed scepter, given to him by Thanos, containing the Mind Stone, and he used the Tesseract (Space Stone) to open the portal for the Chitauri. But the Avengers in the present know that, eventually, Thanos will claim those cosmic artifacts by force, ultimately allowing him to carry out his plan to restore balance to the universe by wiping out half of all life. So, they seek to secure those Infinity Stones for themselves.
Leaked LEGO sets already teased Endgame will pit the heroes against the Chitauri once more, so it stands to reason this scene is where they lock horns. It's worth noting Marvel recently, and controversially, clarified that Loki was corrupted by the scepter, so it stands to reason Thor could free his brother from mind control by revealing Thanos kills the trickster in the future, and maybe even gain another ally in the process. But New York's importance isn't just about upgrading the roster, it's about serving as a pitstop before going up against the Mad Titan in the past.