I usually dislike trying to make theories because I'm absolutely terrible at them. But I just saw it again and a theory popped in my head. It's probably stupid as hell but I just wanna write it down and see how it sounds.
So it was established in the movie that these stones are sentient to some extent. They have a will, can communicate, and will judge and curse unworthy users. That would explain why Thanos couldn't just kill Gamora and then bring her back with another stone. The soul stone can't be tricked. It demanded a soul for a soul.
So I'm thinking, maybe in Avengers 4, the heroes defeat Thanos but lose all of the stones except for one - the soul stone. And they are determined to use it to bring back the lives/souls of those that Thanos killed with his snap. But without the other stones to amplify it's power, it takes self sacrifice, a soul for a soul, to use it.
Tony sacrifices his life to bring back his surrogate son, Peter.
Steve sacrifices his life to bring back his best friend, Bucky.
Okoye sacrifices her life to bring back her king, T'Challa.
Wong sacrifices his life to bring back the Sorcerer Supreme, Dr. Strange. (Okay maybe not, Wong wouldn't do that shyt

)
Rocket wants to sacrifice his life to bring back his friends, but he is only one person, one soul.
So Thor steps up for the rabbit and the rest of the universe. Since Thor is a God, his soul is worth a ton, so he chooses to sacrifice his life for all mortal men and women lost during the snap.
And with that the OG Avengers sacrifice their lives for their loved ones, new protectors of earth, guardians of the galaxy, and the rest of the entire universe.
Raggedy ass theory but I tried
