The Marvel Cinematic Universe spent ten years and over twenty films developing the "Infinity Saga" that reached its finale in 2019's Avengers: Endgame, which took place after the climactic events of 2018's Avengers: Infinity War that left the team devastated and the universe's population halved by Thanos and the Infinity Gauntlet.
Endgame attempted to wrap up a decade worth of storylines while fitting in almost every costumed hero introduced up to that point while still trying to set up the next phase of the MCU and send off a few main characters, which made for an incredibly packed film that made a few details more confusing than they need to be and left some fans shaking their heads.
Why Didn't Captain America Lead The Survivors?
Following Thanos' devastating snap that left the universe decimated, Earth's governments were in upheaval and struggling to maintain order, yet when the film jumps ahead five years later, Steve Rogers is leading a support group instead of the Avengers or the rest of the surviving population.
Not ti imply that the work he was doing wasn't important or that Black Widow was a poor leader for the Avengers, but Captain America could have given the survivors of his country the figurehead needed to rally from the disaster.
How Did Lang's Quantum Tunnel Still Work?
Scott Lang was lost in the Quantum Realm in Ant-Man And The Wasp where he remained for five years until the Quantum Tunnel in his van was activated by a curious rodent that happened to walk over the on-switch.
This incredibly convenient rat managed to turn on a device in a van that had been packed full of equipment and moved without anyone touching the button. Then there had to be an active power source left running after five years, as the old van likely wouldn't have had a remaining charge, and the tunnel was shown later in the film to require the van's juice.
How Does The Time Machine Work?
Lang's return from the Quantum Realm revealed the time difference in the miniature universe that would prove to be an essential part of the Avengers plan to save the day, though the actual "specifics" of the time machine aren't clear.
Even the Back to the Future's DeLorean was explained by the Flux Capacitor, yet Endgame basically threw the problem of how at Tony Stark/Iron Man and left it at that. A briefly glimpsed Möbius strip and the Quantum Tunnel are really the only acknowledged components of the time machine, leaving a bit too much to the imagination.
How Did They Remove The Aether?
Part of the Avengers time-traveling plans to bring back the snapped victims of Thanos included gathering the Infinity Stones from various places across the timestream before they were destroyed, which obviously included The Aether/Reality Stone.
Thor and Rocket Raccoon traveled back to Asgard during the events of Thor: The Dark World with a device to extract The Aether from Jane Foster. However, even Odin confessed to being unable to remove it without killing her in The Dark World, yet the Avengers were able to contradictorily draw it out with a fancy syringe.
How Did Thanos Use The Time Machine?
The final climactic battle of Avengers: Endgame featured Thanos and his army from the year 2014 after they replaced the modern-day Nebula with the still loyal 2014 version and she activated the time machine.
Except, the strict limitations of the "time heist" meant there was a limited amount of Pym Particles available, meaning enough for one round trip for each Avenger. When 2014 Nebula returned to the present-day she used up her final does of Pym Particles, so how was Thanos' ship in the past able to travel through the Quantum Tunnel at all?
Nebula's Time Paradox
Time-traveling characters are bound to deal with the threat of potential paradoxes, and Avengers: Endgame featured a few that have a lot of fans still talking which include the problematic death of 2014 Nebula.
While the film spends some time explaining how timelines can split and create alternate realities, the fact remains that the present-day Nebula killed the past Nebula of 2014 while also bringing 2014 Gamora to the present-day. Did Nebula create a paradox because she killed her past self? Or is the alternate 2014 timeline without Nebula or Thanos now the best MCU reality?
Why Didn't Captain Marvel Use The Nano-Gauntlet?
Following the team's creation and use of the Nano-Gauntlet to bring back the snapped population of the universe, they are forced to play a dangerous game of cat-and-mouse with Thanos' army for possession of the Gauntlet.
Captain Marvel, considered by many to be the most powerful Avenger, takes hold of the Gauntlet and tries to run it through an army, but with so much power at her disposal, why didn't she try to use it? Both Hulk and Thanos survived its use, so she could have potentially prevented Iron Man's sacrifice by using the Gauntlet instead, despite Doctor Strange's prophetic visions.
Where Were They Planning On Taking The Nano-Gauntlet?
The Avengers knew they would have to take the Infinity Stones back to their respective places in time in order to maintain the safety of the timeline (which they technically failed at doing), so when Thanos arrived unexpectedly they tried to get the Nano-Gauntlet to the Quantum Tunnel.
But what was their goal after getting the Nano-Gauntlet to the Tunnel, especially considering there shouldn't have been any Pym Particles left to travel through time? And if they did, Captain Marvel certainly didn't know where to take the stones back in time to close their respective loops, either.
Thanos' Power Levels
When the Avengers and the Guardians of the Galaxy first encountered Thanos in Avengers: Infinity War, he had already gathered a few of the infinity Stones which exponentially increased his power levels.
However, his battles with both teams of Avengers as he gathered more stones still carried weight and felt like there was a struggle happening. Yet when Thanos from 2014 attacked the team without the stones, he seemed to not only hold his own against but almost fared better than his future Infinity Gauntlet-powered self, which seemed somewhat unbalanced.
Captain America's Time Paradox
Nebula wasn't the only character to create her own paradox, though fans are still debating the unique time paradox created by Steve Rogers when he traveled back in time to return the Infinity Stones and stayed in the past to live his life with Agent Peggy Carter.
His decision to grow old in the past made fans question how a hero like Captain America was able to sit by and ignore his knowledge of future events. There is also the matter of his iconic shield that he hands on to Sam Wilson/Falcon, which was destroyed by Thanos. Did Cap steal it from the past, get it from an alternate timeline, or none of the above?