Spoilers ahead for Avengers: Endgame. Avengers: Infinity War left MCU fans with more questions than answers, and Endgame adequately answers most, while also delivering impactful emotional scenes that allow fans to stew in their feelings for a while too. But there are still a lot of topics and loose ends to theorize about, especially now that there might be multiple timelines in the MCU, post-Endgame. You'd think that a movie that lasts a solid three hours would find the time to neatly tie up all storylines, but the element of time travel opens up a lot of doors.
Scott, who proposes time travel in the first place, apparently got all of his ideas about how it works from movies like Hot Tub Time Machine and Back to the Future. Bruce goes so far as to tell him that these movies are totally unrealistic because, in reality, time travel to the past wouldn't affect the future. That doesn't mean that the Avengers can just go back in time and change things without any repercussions. As the Ancient One explains to Bruce back in 2012, changing events from the past will lead to alternate timelines, unless the stones are returned to exactly the moment they were taken. The only thing that the Avengers have to do, then, to ensure that those alternate timelines collapse is to return back in time again to return all of the Infinity Stones.
A few things do go awry during the time heist though, including Loki stealing the Tesseract and then teleporting to another location. In doing so, Loki theoretically creates an alternate timeline which takes place in an alternate reality in which he doesn't return to Asgard with Thor after The Avengers. Steve and Tony have to go back to 1970 to find the Tesseract again, after Loki swipes it, so Steve would have had to return it to that moment, not 2012. As many other sites have pointed out, this alternate timeline could serve as the setting for the upcoming Loki TV show on Disney+ which Marvel is producing, so that apparent mistake may not be a mistake after all.
But Loki stealing the Tesseract isn't the only alternate timeline that gets away from the Avengers. There's also the new timeline which Steve Rogers (Chris Evans) initiates at the end of Endgame when he goes back in time using the Quantum Realm to return each of the Infinity Stones to the times and places when the Avengers stole them earlier in the film. Steve evidently decides to go back even further into the past to the 1940s so he can live his life with his true love Peggy Carter.
Steve somehow hops back into the main timeline as his aged self from the other timeline, which throws another wrench into the whole plot, because how did he do that? Did he simply show up to the lake where Tony's (Robert Downey Jr.) funeral was being held because he knew that they would be there? Anthony and Joe Russo confirmed to Entertainment Weekly that Captain America aged naturally in an alternate timeline, but they left those other questions unanswered.
There's also an alternate timeline created by Nebula's arc. Two Nebulas exist in Endgame, a 2014 Nebula and a present (for Endgame), 2023 Nebula. The 2014 Nebula manages to bring 2014 Thanos to 2023, as well as a 2014 Gamora, so then in Endgame when Thanos' family time travels to the future, an alternate 2014 timeline is created as one without the evil alien family. What a great timeline that is, really. Seeing as how the 2023 Nebula kills the 2014 Nebula, that alternate timeline that chronologically takes place in the past then exists in a world in which Nebula is dead.
Could Thor taking Mjolnir from the past might initiate an alternate timeline as well? Endgame's screenwriters, Christopher Markus and Stephen McFeely, recently spoke with the New York Times and explained that they didn't think too much about that kind of detail.
"I think we’re leaning on, when you just take a baseball mitt, you didn’t ruin that kid’s life. When you took Mjolnir, we accept that that movie happened. Because time is irrefutable," McFeely said. Markus added, "You can make any number of what ifs. The Dark Elves would have arrived, intending to get the Aether. It’s what they came for and it was no longer there."
Well, considering that the people who wrote Endgame choose not to dwell on all of the possible alternate timelines that the movie created, it may be best that you don't either. Then again, what else are you going to do until Spider-Man: Far From Home comes out?