The biggest surprise in Avengers: Endgame might have had nothing to do with Thanos. The Phase 3-ending film contained no post-credits scene. That's about to change, however, when Endgame gets a limited-time rerelease in theaters on June 28.
This news came courtesy of Marvel Studios co-president Kevin Feige, who told Screen Rant that a new version of Endgame was going to hit theaters with a couple of new scenes that weren't in it the first time you saw the film. "If you stay and watch the movie, after the credits," he said, "there'll be a deleted scene, a little tribute, and a few surprises." The way he describes the scenes and that they weren't included in the original release make it sound like they're a little closer to DVD extras than the major plot points you get post-credits in a film like Ant-Man and the Wasp.
Feige said it will hit theaters just a week before Spider-Man: Far From Home. He didn't say if the post-credits scenes will be tied to that film, but the first trailer for Far From Home functioned as a sort of post-credits scene when Endgame was first released. The two films also serve to close out the MCU's Phase 3, with Peter Parker still dealing with the death of his mentor at the start of Far From Home.
He explicitly noted that this isn't an "extended cut," but it sounds like there will be more than a throwaway joke of a post-credits scene like you found at the end of the first Guardians of the Galaxy.
Another reason for the rerelease may be that Endgame is within striking distance of Avatar to become the highest grossing film of all time. Endgame has raked in more than $2.7 billion worldwide, which is about $45 million shy of Avatar. The latter got a rerelease of its own during its first theatrical run, pulling in an extra $10.7 million, per Screen Rant. It would take a pretty significant rerelease for Avengers to catch up. Maybe they'll bank on giving sexy Hulk a little bonus screen time.