One theory speculates that the characters who got dusted at the end of Infinity War didn’t actually die but were instead transported to an alternate dimension. The same theory predicts that Endgame will take viewers into this parallel universe, which will then function as the primary setting for future stories.
This makes a certain amount of sense. After the Thor, Iron Man and Captain America film trilogies, the next phase of the MCU is expected to focus on a new and more diverse lineup of heroes. Meanwhile, Black Panther and Spider-Man, who both died in Infinity War, already have solo sequels confirmed, while the characters left alive, including Iron Man and Captain America, are coming to the end of their contracts with Marvel. Leaving Tony Stark, Steve Rogers and the other original Avengers behind in the old universe would wipe the slate clean and allow new characters to take on their superhero titles moving forward. fans already believe that the next Captain America will be either Falcon or the Winter Soldier.
Starting Phase 4 in a brand new reality would also provide a handy in-universe explanation for the imminent introduction of the X-Men, who have previously been excluded from the MCU due to rights issues but were recently acquired in the Disney-Fox merger.