However, the characters who had been snapped out of existence by Thanos find themselves back on Earth at roughly the point where they left. They have vague memories of what happened at first, but these memories quickly fade when they’re faced with new challenges and pitfalls. In this new timeline, James Buchanan Barnes was picked in World War Two to get the Soldier Serum and he became Captain America back then. His adventures didn’t make it to front pages however, because Bucky manages to accomplish things differently from how Steve Rogers handled them. Bucky shunned the limelight. His sidekick was The Synthetic Man. See The Original Human Torch easter egg in Captain America First Avenger. The Red Skull didn’t have the infinity stone but he’s resourceful. I’m sure he could find some other way to power his weaponry. Events transpire relatively similar to what we know, but with some significant and insignificant changes. Part of the fun of phase four is exploring what those changes are. Nature abhors a vacuum. When this Captain America is found in the ice decades later, it’s Falcon and the US Military who finds him and not SHIELD. Both SHIELD and Hydra are curiously absent from this variant of Earth. At least on the outset. Instead other forces are at work in the shadows. Perhaps AIM. Perhaps The Hand. Nature abhors a vacuum.
My working speculation on Avengers 4 is that in the third act of that film, the core MCU Avengers (Stark, Rogers, Romanov, Barton, Banner and Thor) will have to sacrifice themselves to save not just the universe they know, but the Marvel Multiverse. They face off against cosmic entities tougher than Thanos that seek to utterly destroy their universe entirely since Thanos made such a mess of it, but the Avengers’ sacrifice means that rather than total oblivion the MCU universe that has lost half of all carbon based life is rebooted. It’s a whole new universe with a new Earth in which the stories of the past ten years have been erased. Captain America, Iron Man, Black Widow, Hawkeye, Thor and The Hulk never happened in this universe. SHIELD and Hydra are also noticeably absent. Future movies would take place in a universe a little bit more like the one outside our windows. Superheroes are a relatively new concept on Earth once again, if they’ve happened here at all.
Spider-Man returns to Queens and nobody remembers the snap or anything. None of that happened in this version of reality. Some of Parker’s friends are the same and some are not. Spider-Man Homecoming happened, but without Tony Stark. Peter remembers the original Avengers, but the memories start to fade as he embraces this new reality. I don’t know what the next Spidey movie is, but the less its connected to Avengers 3 and 4 the better. If Spidey ever has to be reminded, he only needs to visit Stephen Strange, who does remember everything and will never forget it. Stephen knows of the original Avengers’ sacrifice, cuz that was the one solution he saw in the time stone before he gave it to Thanos. Since those guys don’t exist anymore, he assumes it worked out. Doctor Strange Two should address this multiverse conundrum. Now that Stephen knows it exists, he’d seek to protect this realm from further tinkering by the cosmic entities. At least he’d try. This would allow the writers to explore some other cosmic entities like The Tribunal. However, Strange also has other more domestic problems like Baron Mordu. T’Challa also remembers the part he played in Avengers 3, but tho a battle against aliens is remembered in Wakanda, T’Challa notices others forget the more cosmic details. His connection to his ancestors and magic shields him from total amnesia, but what’s past is past and he looks towards the future for his people and his country. Black Panther Two should only have a passing mention of Avengers 3&4 events in the first act and quickly move on to more pressing matters.
One problem with this theory of mine is Black Widow. While she’s only 34 years old, Scarlett Johansson can play Natasha Romanov in her youth for the Budapest story, but can Jeremy Renner do Clint Barton? If all the original Avengers have to sacrifice themselves, then these characters don’t exist anymore. If they are spared and return to Earth, they do so in a world without SHIELD or Hydra. What would Natasha Romanov’s life have been like if Hydra had not been behind her early recruitment at The Red Room? If Clint Barton had never worked for SHIELD, would he still be Hawkeye? Would he have been trained under the US Military? Would Romanov still be The Black Widow? Would she still be playing both good guy and bad guy? How much red would there be on her ledger? Would Hawkeye have ever gotten married and settled down? Both of them could essentially be a blank slate now. We could start completely from scratch with new actors if we wanted, or we could tell the story of two ex-spies who have memories of a universe that was replaced with one they do not recognize. Perhaps they decide together that SHIELD needs to exist in this reality, and decide to build it from scratch themselves. Many possibilities here but none of them would be the Black Widow movie that we wanted a decade or so ago. Is that a good thing or a bad thing? Dunno.
Thor could be rebooted as Jane Foster. Hulk could be rebooted as Amadeus Cho. Guardians of the Galaxy could use all the old characters or entirely new ones, or a combination of the two.
Of course, none of this will transpire. Whatever happens it will be nothing like I imagine today, but it’s fun to speculate.