3. ABSURD: A NEW UNIVERSE WITH THE SNAPPED HEROES
There is a prevailing fan theory floating around right now that the snap did not actually end half of all life in the universe. That is to say, what we were witnessing as we watched all of our favorite heroes fade away was not, in fact, the end of them, but rather a movement of these characters from one universe to another.
This theory posits that all of the people who got snapped will now exist in an alternate universe, where the remaining half of people who got to live have never existed. This theory is patently absurd because there is no way Marvel would ever be able to continue telling stories without acknowledging the past. Creating a new universe where there was never an Iron Man or a Captain America just wouldn't work.
Does that mean that Thanos can't eventually see the error of his ways? Maybe not! Some fan theories posit that Thanos might actually be able to redeem himself in Endgame. After all, he must be feeling some remorse about trading Gamora for the Soul Stone, and the scene in the Soul Stone realm when he confronts Gamora as a child has to mean something going forward.
There is a very intentional scene inserted into the Avengers: Endgame trailer and it is that of a compass with a photo of Peggy Carter inside. The compass is pointed squarely at her image and there is only one person in the world to whom this would hold any significance: Captain America.
Fan theories have, of course, suggested that Cap will meet his end in the finale of Endgame, but some of them go even further, suggesting that, like the Soul Stone sequence from the previous film, Steve will be reunited with Peggy and that they will finally have that dance that they waited decades to have.