What do you think will happen in Avengers 4 and how will Thanos be defeated?
(This is what I think will happen in Avengers 4)
What do you want to see in the Hulk vs Thanos rematch in Avengers 4?
Before a "rematch" of the fight, there is an important plot point that must be resolved: where was Hulk in the bulk of Infinity War?
We simply can have Hulk appear and smash Thanos. We know Hulk is not afraid of him. I'm guessing the problem lies in that he is sick of saving Banner's ass when the chips are down, and then, when he does emerge, he is reviled! He's saved Earth, but is feared. Banner is lauded for his knowledge. It's why he took the Quinjet and left at the end of Ultron.
On Sakaar, Hulk finally found what respect and admiration feels like, and, it felt good! No longer is Hulk going to let puny Banner order him around, regardless of the stakes.
The plot is going to have to solve this dichotomy dilemma. Since Captain Marvel is two fused beings, my bet is she will help separate the two, or at least facilitate a dialog between Hulk and Banner. I'm hoping we'll get a "professor Hulk" (Bruce's consciousness and intelligence while in the Hulk body). The promo art seems to have Hulk in a nice bodysuit that seems it would have enough elasticity so both Bruce and Hulk could wear it. "They" could decide when it's appropriate to change into either persona.
With this incredibly important plot point resolved, I'd like to see Hulk deliver one of his patent-pending smash punches to Thanos square in the face. Even better: the rag-doll "puny god" move to Loki in Avengers 1.
I'm sure many of our heros are lining up to get a shot at Thanos. There are those who really deserve a first shot: Nebula, in particular. That said, there are usually a number of Avengers in on the defeat of a villain: Thor, Vision and Iron Man had Ultron Prime on the ropes when Hulk knocked him clear across town. That's a scene I'd like to see with Thanos! Maybe Nebula can get the kill shot when Hulk flattens the goon.
A few things that might happen (or just things I'd like to see...)
Since he has all the Infinity stones, Thanos will have to be massively nerfed for anyone to stand a chance against him (the directors have hinted that he will be.)
Suffering from hallucinations and nightmares of the people he's killed, and the damage to his arm spreading, he goes to Eitri and demands to know what's happening to him; Eitri tells him that no mortal should ever wield the Infinity stones. After the snap, the Gauntlet has become fused to his arm, and every time he uses any of the stones, it will further damage him mentally and physically until he loses his mind and dies. In desperation, Thanos orders Eitri to cut his arm off, but the Gauntlet has a mind of its own and stops this from happening.
It sounds like there will be a time travel plot as well: say, Cap, Ant-Man, Rocket, Hawkeye and a few others travel into the Quantum Realm to change history and retrieve a different set of Infinity stones. To keep Thanos's attention diverted while that's going on, the more powerful Avengers (Thor, Captain Marvel, Hulk, Wong, Nebula... possibly Stark in a new armour if he's not in the Quantum Realm) engage Thanos directly when he is at his most vulnerable, hoping to force him to use the Gauntlet as much as possible.
So the theory im pretty set on is as follows...
Thor will use stormbreaker to find Tony & Nebula they will all regroup on Earth and meet Captain Marvel. They will find Antman and try to use Hank's quantum realm technology to travel back in time to collect the infinty stones before Thanos could ever get them (thus reversing the events). Obviously Thanos will work this out and try using the stones to chase them down. This will be mainly focused on the original avengers.
I imagine the final showdown will take place in Wakanda with all the other heroes starting to come back from the dust to defeat the mad Titan. (The Avengers will have had to have stolen / destroyed one of the stones first).
*With time travel what you do in the past takes time to set in the future which is why just stealing one stone would take to long for things to change and run the risk of not reversing the exact timeline. Also to defeat Thanos they need to take all the stones.
Without having been looking for clues on the World Wide Web of freedom, my guess is that the film's ending will be somewhat unresolved similar to the end of Infinity War itself.
I believe a grey area of "continuity" shift will occure leaving open ended questions about Iron Man (perhaps Iron Man as we know him dies but RDJ from a different time or timeline with a different set of memories is introduced, leaving things open on the character's Participation in future MCU films). The reason I speculated on RDJ first is because his personal and professional Brand as Tony Stark not only kicked off the MCU, but the very brand tenents is "cool, slick and in tune" with the audience resides with him above other actors. His armor is always changing, giving life to new licensed merchandise . He is a fixture.
I think Captain MarVELL's (Jude Law) backstory with Thanos emerges in the Captain Marvel (Brie Larson) film. With Ronin coming back in this flashback film from the 1990's Feige will try to set the stage for next phase of Marvel films, which are said to cosmic focused. With the successful shift from "medieval" to Talea cosmic battles of glory in Thor Ragnorak, look for Feige to push the concept of merging the earth focused MCU into the larger story canvas of the cosmos.
Jude Law's MarVell will mentor Brie Larson and we wi be vested in his character going into Avengers 4. In TH comics, MarVell and Thanos's rivalry is as epic in scale as Dr. Doom and Reed Richards. I think he and Nebula will play a more direct role in Thanos's fate, after expected large scale battles with the Hulk, Ant Man and Brie Larson's Marvel have taken place.
In the comics, it was Nebula who removes the Gauntlet from Thanos's arm. But since the Gauntlet is fractured after the snap, I believe the time travel sequence ends up with the fully stocked gauntlet from a different timeline, in which Feige has planed doe the transition of certain Marvel characters to their next phase version. Feige did hint that many of not all of the chats had that died will remain dead (as defined from thas continuity and timeline as we have witnessed thus far). But as money and already laid out plans for Dr. Strange, Guardians of the Galaxy and Spider-Man sequels, he wouldn't be lying about The dead staying dead if he plucked similar versions of these Characters from a diverged timeline.
I think Thanos will be banished to the Soul Stone a long with the death of Jude Law's MarVELL and he will take the place of the Red Skull as the Soul Stone's game show host. It would be poetic for him to reside in the place of his greatest heartbreak when he killed Gamora to acquire the Soul Stone.
I would also conjecture that Spider-Man gets augmented with Miles Morales being the second spidey. His uncle was introduced in Homecoming for a reason and after starting the THIRD franchise of Spider-Man films, Sony needs more character assets, properly crafted by Feige, to add to its Spider-Man film licensing portfolio.
What an original question! Thankfully, I know exactly what happens in Avengers 4 - The Chimichanga Game.