Lots of people have been saying that the ones who were snapped away by thanos are now trapped in the soul stone like they were in the comics. However, Marvel never steals from the comics directly ; they always only serve as inspiration. In addition, when Gamora was killed and her spirit trapped in the soul world in exchange for the stone, her body did not turn to dust, so that is unlikely to be what's happening to the other characters.
Many people also speculate based on set photos that Ant Man has time travel tech that brings the OG Avengers back to the events of A1. I don't think this is true. This will invalidate the Time Stone and cheapen the plot by introducing a deus ex machina. Kevin Feige and Russo brothers are too conniving as storytellers to allow that.
Instead, I think whatever will undo Thanos' actions is already known - we just haven't pieced it together yet.
I strongly suspect that while Thanos succeeded is making half the world "cease to exist", he didn't quite "kill" them. Instead, with the power of the space, reality and mind stones he zapped them out of existence in this universe... and sent them to a parallel universe instead.
The idea of a multiverse was already introduced in the Dr. Strange movie, and was hinted at by Kevin Feige as a focal point for the MCU.
Ant Man dealt with the Quantum Realm where time and space becomes irrelevant. Marvel hired a quantum physics advisor to consult for the franchise. A theory in theoretical physics deals with "daughter universes", where every choice you make (and don't make) lead to the creation of another universe (google it).
There is a reason why Ant Man and the Wasp and Captain Marvel, the only two movies that come between IW and A4, have to do with the Quantum Realm.
I believe that in searching for Hank Pym's wife in the quantum realm, ant man and co. Found out that the quantum realm provides doorways to alternate universes. Hank then creates the devices needed for them to safely enter these alternate universes. (Captain Marvel might or might not be trapped in one of these universes for some reason - which conveniently explains her absence).
Ant Man then lends the OG Avengers the quantum tech for them to travel to a parallel universe where the NY attack has not happened yet or has just ended (explaining leaked set photos showing them on NY set), which might allow them to retrieve the space and mind stones from Loki (and the other stones latee), so they can create another Infinity Gauntlet to take on Thanos. This has happened in some form in the comics (two infinity gauntlets colliding).
Zoe Saldana (Gamora) accidentally let leak that Avengers 4 migjt be called Infinity Gauntlet. Russo brothers said A4 title is a spoiler.
I strongly suspect that A4 is called Infinity Gauntlets.
This would also allow Marvel to explain the X men's absence should the Fox deal go through.
Thoughts?