Assuming Mysterio can be trusted.
A brand new Spider-Man: Far From Home trailer dropped a serious bombshell: the MCU has a multiverse, apparently, and that's where Mysterio is from. Maybe. We've got some conspiracy theories about just how legit that is. But either way, if we assume it's true, it would seem that Thanos' snap did more than just decimate all living creatures in the universe--it tore a hole in reality itself, and now we're seeing the consequences.
The implications here are wild--maybe even more insane than the implications of all those potential alternate timelines left in Avengers: Endgame--and could spell out the future of the MCU as we move into the mysterious Phase 4.
Let's break it down.
What does a multiverse actually do?
If you've seen Spider-Man: Into The Spider-Verse or if you've ever picked up a superhero comic, you already know the answer to the basics of this question. A multiverse is a system of alternate Earths all occurring at the same time in the same place as one another, separated by different dimensional vibrations. Alternate Earths have all sorts of differences from one another--everything from very slight changes (food brands being different, TV shows being canceled and renewed--you know, normal stuff) to major ones (like whole worlds populated by talking, cartoon animals.)
The function of a multiverse in many comics is to allow for unlimited avenues in storytelling and to explain away various plot holes and continuity problems that come from decades-old superheroes with constantly changing, constantly evolving lore. Multiverses are as utilitarian as they are mind bending.
Marvel's relationship with its own multiversal system is a little unique as of late. Thanks to the Secret Wars event of 2015 (not the one from the 80s, or the one from the early 2000s), a major part of the multiverse was condensed into one new Earth which brought characters from the once-isolated Ultimates universe, Earth-1601--where Miles Morales is Spider-Man--to the "main" Marvel universe, Earth-616. The process erased most everyone's memories of their previous, individual Earths, meaning that almost no one knew there was anything different about the new, amalgamated reality that had been made.
Except, you know, the handful of people who did remember what happened and--well, as you can probably guess, things were a little more complicated for them when it came to navigating the new status quo.
The point being: Marvel's history is rife with multiversal potential and basically free from rules. By introducing a multiverse, the MCU is effectively giving itself carte blanche to really start playing fast and loose with its own established history and continuity.
What does that mean?
With multiple Earths now in play, one of the biggest things the MCU can now do is "resurrect" both heroes and villains who have been taken off the board. There's now a chance we could see Tony Stark again--a different Tony Stark than the one we've dealt with in the main MCU, sure, but still Tony Stark. We could see an alternate reality version of the Skrulls, this time as a group of more familiar conquering aliens. There could be a world out there where Steve Rogers is still an active hero.
There also could be a world out there that literally is the Ultimates universe, meaning a way for the MCU to introduce a live action Miles Morales and have him make the jump into the next phase without taking Tom Holland's Peter Parker out of action. We all saw how things ended for Black Widow in Endgame, even though we know that she's reportedly still got a solo movie in the works--a multiverse means that we may not be stuck in prequel mode for that particular outing afterall. The movie could be focused on any Natasha from any version of Earth.
This actually tracks with the What If? Animated TV show announced for the Disney+ streaming service. Previously explained as "alternate" takes on major MCU moments in the spirit and style of the What If? line of comics, it would make sense for the show to actually take place in different corners of the multiverse where big events went down in totally different ways.
At the end of the day, a multiversal system is a pretty exciting prospect that takes the guard rails of the linear MCU as a whole. Of course, it also has the potential to make things even more complicated--we still don't actually know how the whole timeline thing works in the post-Endgame world--but the limitless potential of an endless list of alternate Earths certainly feels like an exciting road to travel for Phase 4.
Of course, there's also the possibility that the multiverse in Far From Home isn't what it seems. Quentin Beck is not only a villain in the comics, he's a fantastic liar known for staging elaborate illusions and tricks to defeat his enemies, so it might be best to take everything about him with a grain of salt for now. There's a very real chance that this could all be part of his game.