When Avengers: Endgame decided to fix the "snap" by regathering the Infinity Stones using a Time Heist, it was problematic. Not only did they have to introduce the concept of time travel into the film, but they also created their own system of rules; more based on the hypothetical science behind time travel than anything we'd seen previously in films like Back to the Future.
However, while they laid out their own rules for time travel in the film, they also then broke a whole ton of those rules with the belief that all would be set right if the Stones were returned after the Time Heist. As fans know, that didn't actually quite happen the way it was supposed to, and we might have seen a number of new timelines created in the MCU. Some of these potential timelines have similar counterparts in the comics, which we'll explore a bit further today.
10 IRON MAN'S TIME MACHINE
Ant-Man's Time Heist plan would never have worked if Tony Stark wasn't able to figure out time travel lickety-split. There are a number of time travel devices in the comic universe, most notably Doctor Doom's Time Platform and Kang the Conqueror's time-ship, but Tony Stark has built his own time machine before as well.
In Mike Grell's "In Shining Iron" storyline, an Iron Man helmet is dug up in an archaeological dig, just as Tony Stark finishes building his time machine alongside Pepper. Tony, being Tony, rushes off to the past to test his machine and discover the origins of the helmet, which finds him in Medieval times fighting witches and dragons. Talk about an awesome test run for the MCU time machine.
9 HAIL HYDRA
"Six Stones, three teams, one shot." Captain America's pep talk helped bolster his troops, but his team in 2012 New York should have listened better, because... well, things didn't go smoothly. With Loki escaping from this timeline with a Space Stone, that Stone's absence presumably allows all of these timelines to form in the MCU.
Cap himself caused a number of dangerous issues, both by announcing his fake allegiance to S.H.I.E.L.D./Hydra agents Sitwell and Rumlow to gain possession of Loki's scepter and then leaving his past self under the Sceptre's mind-control. Sitwell and Rumlow could use this to their advantage to turn Rogers into Hydra Supreme, as seen in the Secret Empire storyline..
8 SENTIENT ARMOR
Tony and Scott's plan to cause a distraction in New York 2012 ended up shorting out the arc reactor in Tony's chest. While Tony of the future fixed his heart condition, the cardiac arrest Tony experienced in 2012 could have inspired him to fix his heart in a more dramatic fashion, as he did in the comic storyline following a lightning bolt to the armor that gave him a heart attack.
Tony had previously redesigned his armor to help with some health problems he had been having, and the lightning bolt not only damaged Tony's heart but also gave sentience to Tony's armor, thanks to an unknown Ultron virus as well as Tony's inability to protect the armor from Y2K... seriously. Tony's sentient armor turns on its creator, though eventually sacrifices its own artificial heart to replace Tony's.
7 AVENGERS/DEFENDERS WAR
Loki's theft of the Space Stone during the Avengers' Time Heist presumably changed the course of that timeline entirely and allowed for the other mounting issues to turn into their own problems as well. As it stands in the new New York 2012 timeline, Loki has escaped with the Space Stone and it'll likely be up to the Avengers to find him.
On that same note, as per the Ancient One, that timeline is now without the protection of the Stones and about to be assaulted by dark forces, which include the dreaded Dormammu. Dormammu and Loki have teamed up before during the iconic Avengers/Defenders War, and things might not turn out so well in what is quickly becoming the darkest timeline.
6 DARK REIGN
Loki holding the Space Stone in the 2012 timeline is set to be further explored in the upcoming Loki streaming series on Disney+, and as we've previously explored, there are a number of directions this new timeline could take. Having seen the Avengers in action, it's entirely possible Loki decides he might now need his own powerful group to work alongside.
5 LOKI'S INFINITY WATCH
Of course, as we've seen in Loki's appearances after The Avengers in 2012, he does have some redemptive moments in his future. With Loki now in possession of the Space Stone in his alternate timeline, it's entirely possible he could tap into those sometimes altruistic tendencies for an alternative to the Dark Reign option.
As seen in the latest Infinity Wars event, Loki was able to gather the Infinity Stones and place them in the hands of a new Infinity Watch. While Loki's plans for the Watch ended up centering around more trickery, perhaps we'll see Loki use his Space Stone to gather a new Infinity Watch in order to defeat his former partner, Thanos?
4 ASGARDIANS OF THE GALAXY
Cap's team in 2012 isn't the only one to encounter problems, as things don't go great on Morag in 2014 either. Nebula's presence also allows 2014 Thanos and crew to introduce a huge time paradox into the new timelines when they travel to the present/future of 2023, only to be wiped out by a snap of Tony's Nano-Gauntlet.
So, without a Thanos, Gamora, or Nebula in the original timeline, the situations that would have created the Guardians of the Galaxy now no longer occur. So, who could step up to fill that void? Look no further than Asgard, who once assembled their own team of heroes called the Asgardians of the Galaxy. The team featured Angela, Valkyrie, Throg, Frog of Thunder, and more fun characters that could have played a big role in their new MCU timeline.
3 NEBULA'S INFINITY GAUNTLET
Unfortunately, that paradox would continue to cause issues, as present-day Nebula was forced to kill her past self, making her own existence in any timeline questionable. We'd have to imagine that would cause some kind of problems down the line, and we've seen the extremes a dying Nebula can go to.
During the original Infinity Gauntlet storyline, Nebula was reduced to a state of eternal undying by Thanos. She managed to take control of the Gauntlet and restore herself, though her use of the Gauntlet brought the entire cosmic pantheon against her, which would be devastating to this already damaged MCU timeline with no Adam Warlock or Silver Surfer to save the day.
If the time paradox created by 2014 Thanos resulted in the Guardians of the Galaxy never forming, that too would have an unintended consequence that could create a potentially dangerous timeline. If the Guardians of the Galaxy never formed and Peter Quill was left alone to steal his way through the galaxy, it's likely his father Ego would have found him without much trouble.
Without the Guardians to help uncover the truth of Ego and his plans, he might have succeeded in spreading himself across the universe with Quill's unwitting help. The Tales of Asgard stories introduced an evolved form of Ego known as Super-Ego, which would have consumed the entire galaxy if it wasn't for an army of Celestials.
The final component of the 2014 paradox is the ongoing survival of 2014's Gamora in the MCU's current 2023 timeline. While we know that Gamora eventually turns toward the side of good, which she began during the events of Endgame, there still exists the capacity for evil, as we've seen in the recent Infinity Wars comic event.
In that series, Gamora is driven to reclaim a missing part of herself that exists in the Soul Stone, and she is manipulated into becoming the assassin known as Requiem. Requiem not only gathers the Infinity Stones but she uses their power to balance the universe in her own unique way, which could spell trouble for the current timeline of the MCU.