3. Time Travel
This is one of the most popular theories circulating around the Internet, for two reasons. First, Ant-Man and the Wasp deals directly with the Quantum Realm, which has some connection to the space-time fabric. As Hank Pym says, in the Quantum Realm, “time has no meaning.” The last time we saw Ant-Man and company, the device they had built to access the Quantum Realm was ready and functioning, and Ant-Man himself was stuck inside the Quantum Realm, on account of the rest of his team disintegrating thanks to the Snappening before they could bring him back.
Second, on-set pictures have shown the Avengers wearing some sort of a device around their wrists. These could be time travel devices not unlike what Cable wore to travel through time in Deadpool 2.
One more note: in those set pictures, Tony Stark looks older than he does in the Endgame trailer. It’s likely that he survives his trip back to Earth and gets into the time-traveling business a couple of years after. Theoretically, this means the Avengers could travel back through time to before the Snappening happened and somehow prevent it.