The short Answer
There have been many theories about Avengers: Endgame, But only a few seem to be more accurate than all the rest.
As to the question: Who will defeat Thanos? it is a bit complicated.
I’m not sure anyone actually really wants to know the answer to this but… at the risk of spoiling the film. The short Answer is: Hulk.
The longer answer, however:
A poster on 4Chan released what he claims is the total plot – apparently entitled Infinity Gauntlet. He released the plot of infinity war before it came out and was mostly accurate so I wouldn’t be surprised if it’s actually true, most of the stuff which will end up happening in the film.
It also broadly matches with the upcoming Marvel schedule as it strongly features ant man.
Here is how it goes:
The theory suggest that Iron Man reassembles the Avengers and decides to build his own Infinity Gauntlet to undo everything that Thanos did.
They recruit Ant-Man to help them travel through time and space using the Quantum Realm to retrieve the Infinity Stones from different time periods.
Thanos finds out about their plans and becomes hellbent on stopping them.
The movie revolves around the relationship between Captain America and Iron Man
Captain America and Mjolnir
At one point, Captain America and Thor fight Thanos. Timeline alterations have restored Mjolnir, and Cap wields it against Thanos to allow the others to escape and is killed holding Thanos off.
(Remember when Cap almost moved Mjolnir a little in Avengers: Age of Ultron)
Hawkeye, Thor and Others
At one point, Hawkeye must protect the unfinished Stark Gauntlet from Thanos’ minions. He plays an “instrumental” role in Thanos’ defeat.
Thor’s subplot centers on him assembling an army to challenge Thanos. Captain Marvel joins him.
Nebula’s subplot centers on her efforts to redeem herself. At one point, she fights her murderous past self.
Several MCU movies are revisited and retconned (probably not permanently), such as the Avengers retrieving the Power Stone creating a timeline where the Guardians of the Galaxy never came together.
There’s a pivotal scene between Doctor Strange and a fully CGI character being shot on a secret location, with a skeleton crew, and which takes up a sizeable portion of the budget.
Only two of the original Avengers (Cap, Stark, Thor, Hulk, Hawkeye and Black Widow) meant to survive the movie. Cap dies.
Rebuilding the infinity gauntlet and going after the stones is obviously the only way that the universe gets fixed and all the Avengers come back.
That Captain America is going to die makes sense as the actor has been saying he would be happy to stop doing these films for a while now. A fitting end for him and logical given his self sacrificing tendencies.
Mjölnir coming back also makes sense, everyone wants to see that hammer back. It’s as much if not more of a character than some of the lesser avengers.
The Hulk plot
Hulk’s subplot centers on Banner and Hulk finally merging to become Professor Hulk. He is the one that ultimately wields the Stark Gauntlet against Thanos, losing his arm in the process.
Hulk’s plot makes sense as he took a major step back in Infinity War. Coming back as the main fighter this time with both Banner and Hulk together to settle a score with Thanos, becoming the strongest in the universe seems logical. Definitely a crowd pleaser. I doubt he will permanently lose the arm, given the obviously reduced value of ‘disabled Hulk’ as a Disney franchise. If he can bring back half the universe he surely can fix his arm – if not, I’m sure Tony can sort it out in time for Avengers 5.
Little said in terms of spoilers about captain marvel so I assume a lot will be covered in her new film which comes out about a month or so before this. 4chan will probably release the plot for that before it’s out too.
The title was AVENGERS: INFINITY GAUNTLET at one point apparently but it’s still in flux.
Marvel may respond to some of this and move things around but I would expect most of those things will happen.
I am looking forward to seeing Asgard somewhat restored, all the Asgardians can’t be dead forever.
Strange and the others, Gamora, etc, are probably trapped in the soul stone’s pocket dimension. That is often what has happened in the comics. Dr strange will return.