It seems obvious that many of the heroes who were snapped away at the end of Avengers: Infinity War will manage to return by the end of Avengers 4. How else would there be a Spider-Man: Far From Home or Guardians of the Galaxy Vol. 3?
But it does beg the question of what will happen to Gamora, who wasn't snapped but was tossed from a very high cliff in a sacrificial ritual. That seems a bit harder to come back from.
With all of the hints of Gamora's continued presence, as indicated by the scene in which Thanos speaks with the child version of the character within the Soul Stone, there is an avenue to bring her back. And one Marvel fan thinks he's cracked the code that will be used to do so.
Reddit user hockeytalkie recently posted a fan theory detailing a path for Gamora's return, but that it wouldn't actually happen until Guardians of the Galaxy Vol. 3. Basically, everything in Avengers: Infinity War and Avengers 4 would set up the events in the last Guardians movie.
The gist of the theory states that the Soul Stone and the rest of the Infinity Stones would be dispersed after Earth's Mightiest Heroes manage to defeat Thanos. But, assuming that Gamora's spirit is still trapped in the Soul Stone, the Guardians would fail to rescue her before it travels across the universe.
Much like the comic books, Gamora would spend some time in the Soul Stone, and much like the comic books the artifact would become intrinsically linked to Adam Warlock. The theory indicates that the Sovereign would come into possession of the Soul Stone, once again bringing them into conflict with the Guardians as they attempt to rescue their friend.
With the Soul Stone used to bring Adam Warlock to life, the Guardians would battle him and the Sovereign in the physical realm while Gamora dues with Adam on an ethereal plane, likely convincing him to join their side. But there would also be the dilemma of one of them having to die for the other to live, which could be a central conflict to the film.
All in all, it's not a terrible theory, but former director and script writer James Gunn has repeatedly stated that Adam Warlock is unlikely to be in the film. Was he just blowing smoke? Hopefully we find out as we get closer to the premiere of Avengers 4 and, eventually, Guardians of the Galaxy Vol. 3.