Thanos' snap. The plan requires the team to travel to different points in their timeline to obtain different Infinity Stones.
Specifically, the plan is for Iron Man, Captain America, Hulk, and Ant-Man to visit full breakdown of Avengers: Endgame's time travel rules, the Avengers believe the fundamental rule is that they can't change the past of their own reality.
However, The Ancient One later shows this to be incorrect. The Avengers' actions in the past can still have consequences, but rather than changing the future they create branched realities. She explains that alternate timelines can be created by removing or changing what happens to an Infinity Stone, as they are crucial to the flow of time. These realities can be fixed though by returning the Infinity Stone to the exact moment that it was taken. It is important to note that smaller changes may not create new realities though, as they could've always happened in a linear timeline with time loops.
At the Soul Stone to Vormir in 2014 (however that would work), the Power Stone to Morag in 2014, and the Reality Stone and Mjolnir in 2013. But is it that simple? Based on the logic of Avengers: Endgame (and what's been said by the filmmakers), here's every definite - and possible - new timelines the Avengers still created.
New Timeline #1: Loki Escapes The Avengers In 2012
During the mission in 2012's Battle of New York, the team appears to mess up the timeline in more than one way. The most obvious one was the moment when the Tesseract is knocked to Loki's feet, prompting the God of Mischief to teleport away. This appears to create a still working for Thanos at this time, meaning he could go back to the Mad Titan after his apparent defeat, but still with one Infinity Stone in hand. This would leave the 2012 Avengers with a need to find Loki (Cap returned the Tesseract to 1970, not here).
In this new timeline, Captain America's part of the main mission could also bring some changes. He takes Loki's scepter by saying "Hail Hydra" to the Strike Force team, which would leave those sleeper agents with the impression that he has ed their cause. Additionally, 2023 Cap tells his 2012 self that Bucky is still alive. This all would dramatically change the events of Captain America: The Winter Soldier, if it happened as we know it at all. That said, those changes caused by Cap could potentially be thought of as Loki in-universe as 2012 Cap thinks that is who he is fighting. This would make for a serious amount of retconning, which isn't totally out of the question, but a new timeline where Loki wasn't captured in 2012 has definitely been created.
New Timeline #2: No Thanos (Or Guardians) After 2014
The other new timeline that would be unfixable for Captain America is a new timeline starting in 2014. This is the result of Nebula's memory network automatically connecting to her past self, which alerts Thanos to what the Avengers are doing and what he manages to accomplish in the future. This revelation sees 2014 Nebula steal 2023 Nebula's identity and jump to 2023 so she can allow Thanos to travel to this point in time and attack. However, the movie ends with Iron Man snapping Thanos and his entire army out of existence, 2023 Nebula killing her past self, and 2014 Gamora's status unresolved. This ending for Avengers: Endgame would create a brand new timeline from 2014 and on that would be without Thanos, his army, Nebula, and Gamora (although what exactly with the Infinity Stones enabled this is unclear).
The implications of their absences are huge, even if Cap returned the Power Stone to the temple for Star-Lord to find. Thanos not being around would suddenly allow Ronan to not answer to anyone, and he could have less resistance to level Xandar too. Without Gamora in this timeline, the Guardians of the Galaxy never would be brought together as we know them. This would also allow someone other than Thanos to try and collect all the Infinity Stones. Ronan could do this if he got the Power Stone all on his own, while The Collector would already have the Reality Stone at this point and could continue his quest to get the remaining five.
New Timeline #3 (Possible): Captain America Returns To The 1940s
Captain America clips as many branched realities as he can, but he may still have created another new timeline with what he decides to do next. He chooses to go back to the 1940s to see Peggy Carter, where they share the long-awaited dance. This isn't just a single moment though, as he stays there and grows old with the love of his life before reappearing in 2023 to the Captain America mantle and shield onto Falcon.
Whether or not this arc does create a new timeline, though, is up for debate, to the point where directors Anthony and Joe Russo have a different interpretation of the moment than writers Christopher Markus and Stephen McFeely. The Russos say that Cap always traveled to the past to be with Peggy and thus was her original husband in the main MCU timeline, making this effectively a time loop. The latter explanation goes against what happens with Thanos, which could point to this being a true new timeline full of possibilities for additional Cap and Peggy adventures.
New Timeline #4 (Possible): No Mjolnir After The Dark World
Even though we're led to believe that Captain America returns the Reality Stone and Mjolnir to Asgard in 2013 at the right moment to avoid creating a branched reality, the suggestion that any instance of time travel can create a timeline could lead to a slightly different offshoot. Frigga meeting a future version of Thor could have impacted her death, while Rocket running away from Asgardian guards with the Reality Stone could have other implications. And that's nothing on Thor losing his hammer four years earlier.
New Timeline #5 (Possible): A New SHIELD Timeline In The 1970s
Using this same logic, it is also possible that a slightly different timeline could be created in 1970. Even if Cap returned the Space Stone to Camp Lehigh, his prior actions could result in a timeline where Hank Pym sees some of his Pym Particles stolen (unless he returned those too). This could lead him to believe that S.H.I.E.L.D. or someone else is trying to steal his invention. Other than the possible changes that could come from this, there's not much else in 1970 that could create a new timeline, other than Howard Stark's conversation with a future version of Tony Stark.
What Do These Timelines Mean For The MCU?
The question all these new timelines have left the MCU with is whether or not these multiple timelines will actually have any meaning as the shared universe continues. The MCU has a big enough story on its hands with just the characters and canon that the last 22 films have built, so avoiding the complications of what is happening in other timelines could be for the best. They could just be collateral of the Avengers' actions and never mentioned again.
But, based on the trailer for series on Disney+. Depending on how the MCU continues to progress, these new timelines and the multiverse could eventually be used to bring back characters like Iron Man and Captain America - but with completely different histories.
This could even one day be the key to the Russo brothers directing a Secret Wars movie, which they've repeatedly said would be the one project to bring them back to Marvel. It is too early to tell whether or not these timelines will have a place in the MCU beyond Avengers: Endgame, but there are clearly several possibilities for Marvel Studios to do something with them if they want.
