When we see Tony come back from space and Cap is leading the rest of the team whilst taking control, Cap turns to Tony and says ‘Well, you’re not wrong’. This is a direct quote from Avengers Assemble except it is Tony who says it to Steve. Whilst Tony is trying to fix the broken engine and turbine of the Shield Carrier craft he gives Steve instructions how to handle the electronics of the hangar and which wires to cut. Steve has only just emerged from the ice and isn’t familiar with technology but better yet isn’t a tech genius like Tony so when asked what he sees he simply replies with a bunch of wires and Tony replies ‘you’re not wrong’.
Bringing this arc round in full circle by a subtle one-liner by Steve Rogers in Endgame is significant in showing how far the characters have progressed. The tables have turned. Steve is now fully aware of modern life, is less trusting and more knowledgable whilst Tony is less egotistical and slightly more battle worn as well as willing to sacrifice himself for the greater good.
Avengers: Age of Ultron
We also learn that the scene in Avengers: Age of Ultron where Wanda manipulates the Avenger‘s minds and they each have their own visions, that these visions were in fact showing us the future of the MCU. Thor sees his happiness with his family and friends at Asgard turn to sadness, loss and isolation which later happens in Ragnarok. Tony sees all his fellow avengers, friends, family everyone he cares for die and he is unable to save them. He feels guilt and responsibility as he knows he needs to find a way to protect them, save them and bring them back to life whatever the personal cost which comes to pass in Infinity War and Endgame.
Captain America sees his dance with Peggy Carter get disrupted by bullets, war and death. This happens way back in The First Avenger but in his vision he dances on despite the pain, loss and war he’s experienced and they get their happy ending, which happens in Endgame. Natasha sees herself back in the red room, unable to escape her training, her demons and to wash away the red she so desperately wants rid of. She never escapes her fate but instead like in her vision sacrifices herself to help her friend, knowing she is already doomed as happens in Endgame.
Even back in Avengers Assemble, Loki told Natasha she could wipe away her past and it’s something she is still battling with in Endgame. In Endgame she finally accepts her demons and her fate rather than fighting them, and in doing so helps the avengers win the Infinity War against Thanos.
The Avengers also take turns in Age of Ultron to attempt to lift Thor’s hammer and can’t. Steve Rogers though doesn’t need to prove his worth or that he can lift it and barely tries. In Endgame we finally see Cap wield Mjolnir and accept his true worth, identity and power.
Captain America: The Winter Soldier and The First Avenger
Not only do we revisit an exact scene in The Winter Soldier within Endgame (cue Cap in a lift taking down a bunch of hydra agents) but we also revisit characters like Frank Grillo and Agent Sitwell. Cap‘s iconic line ‘I could do this all day’ featured both in The First Avenger and The Winter Soldier and proves problematic when past Cap is pitched against future Cap who now knows better, not to mention the use of the Red Skull and his fate as revealed in Infinity War.
Thor: The Dark World
It’s pretty obvious that this movie is revisited as Rocket and Thor head back in time to extract the Aether from Jane. However there are lots of other references to this movie too which proves just how important it is to the MCU. It sets all the emotional stakes for Thor and he’s able to deal with some of the past events which trigger his PTSD, which take place way before Ragnarok such as the death of his mother, his break up from Jane, the deceit and loss of trust with his brother and his battle to prove to himself and his father that he is worthy. Not to mention, Thor is still confused about his own identity and trying to find himself. He knows he isn’t a king but he isn’t sure what he is instead.
Thor’s revisit to The Dark World puts these fears and anxieties to rest. He realises he is worthy, his brother did come to realign himself with Thor, he hasn’t let his mother or his father down and the fall of Asgard was inevitable rather than his fault. This burden is lifted and Endgame comes full circle as Thor is able to move on and accept his own identity, whilst treading a new path that isn’t part of traditional Asgardian legacy, accepting his true self and his own worth.
He finally realises being worthy is nothing to do with being King or being a God which is one of the reasons why Captain America is then able to lift the hammer. It’s about finding and accepting themselves and their own worth rather than any external judgement but instead understanding their own identity and importance.
I don’t really need to mention just how important Hank Pym’s discoveries in the Quantum Realm are to Endgame and time travel but it turns out that Ant-Man is one of the most important films in saving the MCU and setting up the finale of the Infinity Saga. He also ups the emotional stakes in discovering that he’s missed his daughters entire years as a child growing up and that his family has become neglected due to his superheroing antics.