The events of Avengers: Endgame provided such a satisfactory conclusion to the first three phases of the Marvel Cinematic Universe that there’s been a brief lull in fan speculation about the future of the franchise. However, with Endgame en route to its inevitable resting place as the highest grossing movie of all time, Marvel Studios isn’t going to stop making movies any time soon - and the future of the MCU beckons once again.
Phase Three isn’t quite complete - Spider-Man: Far From Home will actually be the last film in the set - but it's clear that the saga of the Infinity Stones has come to a conclusion. Many believe that Far From Home will serve to set up the MCU for Phase Four and beyond - and fans are slowly starting to move away from Endgame Easter eggs and plot deconstructions and on to theorizing about the films yet to come.
Some fan theories regarding the future of the Marvel Cinematic Universe are general and overarching, but others are highly specific - and one by Redditor /u/papa_prevail definitely falls into the latter category.
They posit that a scene from Spider-Man: Far From Home in which Peter Parker dons an old pair of Tony Stark’s sunglasses actually holds significant meaning, because Stark’s sunglasses likely contain some high-tech secrets. Specifically, they think Stark’s sunnies will allow Peter to see through Mysterio’s deceptions - and perhaps learn that the giant monsters that Mysterio is fighting are not quite what they seem.
As far as we can tell from the Far From Home trailer, Peter Parker and the majority of his friends disappeared when Thanos snapped half the universe out of existence - and they’ve been absent from Earth for five years. However, not all of Peter’s classmates would have been been snapped, so there are bound to be students that were once younger than him that are now in his grade - and that could include some of his classic comic book friends.
A theory by Redditor /u/Bman1738 posits that Harry Osborn - the son of the Green Goblin and Peter’s best friend in the comics - will be introduced as a new classmate of Peter and the gang. Alternatively, Redditor /u/pacificwisdom theorizes that Miles Morales will make his MCU debut in a similar fashion.
In the trailer for Spider-Man: Far From Home, Mysterio claims that he hails from an alternate Earth in the Multiverse. While the veracity of his statement is highly dubious, it wouldn’t be much of a surprise at all if the Multiverse was coming to the MCU. However, it might not come as a result of Thanos’s snap, but as a consequence of the actions of the Avengers.
According to a fan theory by Redditor /u/jarred_98, the time-travelling shenanigans of the Avengers in Endgame are the actual reason that the Multiverse has come into existence. By creating a series of alternate timelines in their quest for the Infinity Stones, the Avengers inadvertently created a bunch of Earths where things are slightly differently than they are in mainstream continuity - and these new worlds could be explored in future films.
A contemporary storyline in Marvel Comics features T’Challa’s nation of Wakanda entering into a conflict with the subterranean empire of Atlantis, led by Namor the Sub-Mariner, one of Marvel’s oldest characters. Fans have been waiting for Namor to show up since the Marvel Cinematic Universe began, and Redditor /u/jajalool thinks the seeds have already been planted in Avengers: Endgame.
When making a report to Natasha Romanoff, Okoye makes reference to underwater earthquakes, which might seem like a throwaway line. Alternatively, however, it could be foreshadowing Namor’s impending arrival - and setting him up for conflict with Okoye and the other Wakandans, perhaps in a Black Panther sequel. Redditor /u/jajalool says, "Namor will be the next villain for Black Panther. The mention of the earthquake under the ocean was a hint towards it. Namor will be mad his people were lost, and will believe that he is superior to the heroes we have."