The MCU would look very different if Ant-Man didn't return from the Quantum Realm in Avengers: Endgame. After Thanos decimated half of all life in the universe at the end of Avengers: Infinity War, all hope seemed lost for Earth's Mightiest Heroes, with many of their own ranks being turned to dust following Thanos' snap. Avengers: Endgame picked up five years later, showing how difficult the of the Avengers had found it to move on from their unprecedented defeat, though a glimmer of hope was provided when Scott Lang, a.k.a. Ant-Man, returned from the Quantum Realm.
Ant-Man and the Wasp saw Scott Lang enter the Quantum Realm, perfectly timed with the ending of Avengers: Infinity War. After Thanos' actions in Wakanda, Hope Van Dyne, Hank Pym, and Janet Van Dyne, who were supposed to bring Scott back, were turned to dust, trapping Scott inside. Thanks to a convenient rat walking over the Quantum Tunnel's controls, Scott returned five years later with a plan for the Avengers to use the Quantum Realm to travel through time, collect their own Infinity Stones, and bring everybody back. Had he remained stuck in the Quantum Realm, however, Endgame and the future of the MCU would have been drastically different.
6 The Avengers Would Have Lost
The Avengers' defeat in Avengers: Infinity War came as a sobering reality check for the MCU since nobody expected the team to lose so spectacularly. However, this version of events was apparently paramount to saving the universe in Avengers: Endgame, as proven by Doctor Strange's visions when preparing to battle Thanos on Titan. Using the Time Stone, Strange observed over fourteen million possible futures, though only found one instance in which the Avengers succeeded, that being the one which included the Avengers initially losing to Thanos, Ant-Man's return from the Quantum Realm, and the escapades of the Time Heist.
If Scott Lang didn't return from the Quantum Realm in Avengers: Endgame, this would perhaps suggest an earlier deviation from the timeline seen by Doctor Strange, negating the one scenario in which the universe was saved. This would have meant that there'd be no going back; the Avengers would have lost permanently, with no chance of seeing their lost loved ones return. While this might be a bleak realization, it proves how intricate Doctor Strange's plan was for the battle against Thanos and solidifies the Avengers' resolve during the five-year Blip, as well as their determination to bring their family home.
5 Endgame’s Sacrificed Heroes Might Have Been Spared
While Spider-Man: Far From Home acted as the epilogue of the Infinity Saga, setting the stage for the Multiverse Saga, Avengers: Endgame concluded many stories that had been unfolding since 2008's Iron Man. This included seeing several of the original Avengers hang up their superhero hats, paving the way for new heroes to take their places. The first of these came with Natasha Romanoff sacrificing her life on Vormir to allow the time-traveling Avengers to acquire the Soul Stone, and although Scarlett Johansson would reprise the role in Black Widow, Endgame marked her final chronological appearance.
Perhaps the biggest changes, however, came with the departures of Robert Downey Jr. and Chris Evans as Tony Stark and Steve Rogers, respectively. While Evans' Captain America ed his shield to Sam Wilson after retiring in the past with Peggy Carter, it was Iron Man who had the most emotional farewell, laying down his life to eradicate Thanos and his army. All of these sacrifices were possible because Ant-Man returned from the Quantum Realm with the prospect of time traveling, meaning that if he'd remained trapped, the MCU's Black Widow, Captain America, and Iron Man would've been spared, albeit in a much darker world.
4 He Who Remains Would Have Kept Protecting The Multiverse
The events of Avengers: Endgame teased several important plot threads that would be explored in the MCU's Phase 4, most notably the introduction of time travel in the MCU and the idea of creating alternate and diverging timelines. This formed the crux of Loki on Disney+, which featured the 2012 variant version of Tom Hiddleston's God of Mischief being captured by the Time Variance Authority (TVA) and learning the mysteries of the multiverse. Loki season 1 ended with Loki and his female variant, Sylvie, seeking an audience with He Who Remains, the guardian of the Sacred Timeline, who just so happened to be a variant of Kang the Conqueror.
If Ant-Man had never returned from the Quantum Realm in Avengers: Endgame, the Avengers wouldn't have traveled through time and released the 2012 version of Loki from his imprisonment, meaning the events of Phase 4's Loki would never have happened. Loki and Sylvie would never have met, Sylvie wouldn't have had the strength to enchant Alioth and enter the Citadel at the End of Time, and He Who Remains wouldn't have been killed by Loki's more vengeful variant. This would have meant that He Who Remains would have continued to maintain the Sacred Timeline, and the multiverse, or Kang the Conqueror's many variants, wouldn't have been unleashed.
3 Tony Stark Might Have Eventually Invented Time Travel
It was mentioned during Avengers: Endgame that Earth's Mightiest Heroes initially pondered time travel as a plausible solution to finally defeating Thanos but discounted it as an outlandish and impossible idea. It was only Ant-Man's expertise of the Quantum Realm and Tony Stark's genius that allowed the team to succeed, so it's unlikely that time travel would've been an option had Scott Lang not returned. Even so, it's entirely possible that Stark, being the genius that he is, could have someday figured out the solution to time travel, which would have provided the option to embark upon the "Time Heist."
Even if Tony Stark had managed to invent time travel and presented the other Avengers with the option of traveling through time to bring everybody back, this could have taken place years or even decades down the line. By this time, the world would have fully adjusted to the changes after Thanos' snap, and everybody would have moved on from the catastrophic events of Avengers: Infinity War, perhaps meaning that the Avengers wouldn't risk losing everything they rebuilt. The effects of the Hulk's snap bringing everyone back in Avengers: Endgame were devastating to many across the world; this impact would have increased tenfold had even more time ed.
2 The Eternals May Not Have Made Themselves Known
One of the biggest questions that Marvel Studios needed to answer in Phase 4 was the whereabouts of the Eternals during the events of Avengers: Endgame. The new superhero team was introduced in Eternals but was revealed to have been living on Earth for over 7000 years, slowly helping humans to progress technologically and ensuring the growth of the population so that the Celestial Tiamut could emerge. Eternals confirmed that the team was forbidden from intervening in human affairs unless Deviants were involved, which meant that they had to sit on the sidelines and watch as Thanos succeeded in his Infinity War mission.
While the Eternals' mission of expanding the human population would have been delayed for potentially hundreds of years, they also would have had no reason to reunite after going their separate ways in the 1500s. They came back together in the days leading up to the Celestial Tiamut's emergence, which wouldn't have occurred had the Avengers not brought the Earth's population back to life. Even so, it's possible that some of the Eternals could have made themselves known to Earth's Mightiest Heroes, perhaps even offering their help, as such as Sersi, Ajak, and Phastos were shown to have become very attached to humanity.
1 MCU Phase 4 Would Have Been Very Different
If Ant-Man had never returned from the Quantum Realm in Avengers: Endgame, the landscape of the MCU's Phase 4 and beyond would have looked remarkably different. Phase 4 was very much about the universe's residents dealing with the after-effects of Endgame's five-year time jump, despite it perhaps not being mentioned as much as it should have been. On the big screen, the events of Eternals, Spider-Man: No Way Home, Doctor Strange in the Multiverse of Madness, Thor: Love & Thunder, and Black Panther: Wakanda Forever simply wouldn't have happened, as most of the heroes featured in these films were snapped away, and wouldn't have returned.
Phase 4 also saw the introduction of MCU-set long-form shows on Disney+, which would have been changed massively had the Avengers not succeeded in Endgame. WandaVision, The Falcon and the Winter Soldier, Loki, and even What If...? would have been wiped off the map, and the events of Hawkeye probably wouldn't have seen Kate Bishop being mentored by Clint Barton since he would have still been operating as the vengeful Ronin. It is possible, however, that Moon Knight, Ms. Marvel, and She-Hulk could have still become heroes, perhaps forming the basis of a new Avengers team in a continuation of Avengers: Endgame's dark and traumatized world.
