The events of Avengers: Endgame would have been very different had the other half of the Avengers been snapped out of existence by Thanos in Avengers: Infinity War. The storytelling choice to rely on the original MCU Avengers team of Captain America, Iron-Man, Thor, Black Widow, Hulk, and Hawkeye with some help from Ant-Man, Nebula, Rocket, and War Machine was sensible. Acting as a farewell to old friends so that newer ones could return to take over the mantel led to a satisfyingly emotional conclusion to the Infinity Saga.
Following Thanos’ destruction of the Infinity Stones and his subsequent beheading by Thor, five years passed. During that time Tony Stark became a father, Bruce Banner finally took control of the big green rage machine within him, Thor sunk into depression, Steve Rogers started a self-help group, Clint Barton complicated his hero status by becoming a vigilante, and Natasha Romanoff did everything she could to hold the remaining Avengers together. Even so, when Scott Lang gave them the idea for a "time heist" and Tony Stark made that into a practical possibility, their collective desire to put right what had gone wrong and return the people they had lost was too strong to be ignored.
This could all have been completely different though. The nature of Thanos’ plan was that there was no value judgment on who lived and who died in the Snap. The result was random and, despite amounting to genocide on an unfathomable scale, Thanos claimed this made it fair. This raises an intriguing question: how would events have played out if the other half of the Avengers had survived the Snap? With what was already known about these characters it is possible to make an informed guess as to how they would have used their powers and abilities to not only change the past but also lay the groundwork for the future of the Avengers.
It has become easy to assume that Tony Stark/Iron-Man was the preeminent figure in the Avengers. After all, he started the MCU and has often proven to be both the MVP and, in Avengers: Age of Ultron, the cause of the team’s problems. The former happened again in Avengers: Endgame when he used his genius-level intellect to solve the problem of time travel. This wasn’t a problem that any scientific genius could solve, since, in Avengers: Endgame, Bruce Banner/Hulk could only send time through the body of Scott Lang, making him a teen, an old man, and a baby before turning him back to his proper age.