Avengers: Endgame forces Earth's Mightiest Heroes to fight harder than ever before in their attempt to defeat Thanos and reverse the snap. In the end, the heroes are able to undo the effects of the Infinity Gauntlet and restore life back to the universe, but not without a great deal of sacrifice.
After the fact, the people of the Marvel Cinematic Universe see the sacrifice of Tony Stark as the final act of the world's greatest hero, but they also seem to be brushing aside the efforts and pains of Bruce Banner/Hulk. It's ultimately Hulk who did a lot of the heavy lifting to help restore life to the universe, but the world just seems to ignore it. This oversight might seem petty, but it could have dire consequences for the future of the MCU.
The Big Hero
Spider-Man: Far From Home establishes that eight months after the events of Endgame, Tony Stark has become a modern legend. Already the most famous person on the planet, now Tony's the man who gave his life to stop Thanos and his armies once and for all. And sure, that's a heroic sacrifice. He goes out the hero no one thought he'd ever become. But while the world wonders what will become of everything now that the Avengers are no longer a thing, the only one who seems to be truly honored is Iron Man.
This completely undercuts the sacrifices made by other members of the team. Other heroes who died, like Black Widow, receive barely any mention compared to Tony, and no one seems to consider the pain many were forced to contend with. Notably, the Hulk is now a very changed man. He spent Endgame as something different than the giant bruiser he once was. Hulk initially seems to have become much more socially accepted by the world in this new "Professor Hulk" form. By the reaction children had to him, he seems quite popular. One would expect the world to recognize his part in bringing half of all life back.
Get No Respect
It's important to remember just how much Hulk manages to do over the course of Endgame. After spending the rest of his time in the MCU torn between his human side and his empowered form, Hulk managed to find a peaceful center. Professor Hulk has all the power of Hulk and none of the rage, and he's integral in perfecting the time travel they need to utilize to recover all the lost Infinity Stones. He even single-handedly recovers the Time Stone from the Ancient One in the past. When the Stark version of the Infinity Gauntlet is constructed, he's the one to wield it and bring half of all back to the cosmos.
In the process of using the Infinity Gauntlet, Hulk burns out one of his arms. He spends the rest of the film unable to use it, even in the final battle against Thanos and his armies. After the battle, the arm is still shown to be withered, and Endgame director Joe Russo claimed the injury is permanent. Considering how quickly he'd been able to recover from most injuries, the fact he wasn't able to walk this one off shows just how much he sacrificed for the universe. Outside of just the physical damage he sustained, Hulk was also unable to bring back his former lover, Black Widow.
There are no giant murals to Hulk like there are Tony, and he doesn't seem to be getting the credit he deserves for his acts. Those are the kind of things that could make someone angry about a lack of appreciation. And you wouldn't like the Hulk when he's angry.
The Coming Of The Maestro
The Hulk has often been considered a monster. Even after he helped push back the alien invasion in Avengers, Hulk lost a lot of respectability when he attacked Johannesburg in Age of Ultron. This might explain part of the reason why the Hulk isn't so readily accepted as an icon following Endgame. His role in the film may have even been downplayed when compared to the more publically showcased Tony Stark. But that lack of appreciation could easily give way to fear. Far From Home shows just how easy it is for heroes in the MCU to have the public turned against them. And considering his history, it wouldn't be difficult for the public to turn on the Hulk.
After everything he sacrificed, it could easily make him just as angry as he ever was to see a world where he's not respected or appreciated, even when he's proven to be just as much of a hero as former science bro Tony Stark. But now that he has the brilliance of Professor Hulk, he could be significantly more dangerous. If anything, it could even lead to the birth of a different kind of Hulk.
The Maestro is a potential future version of the character. He's one of the most dangerous threats in the Marvel multiverse, with all the Hulk's strength paired with a devious, brilliant mind. This also happens to be a version of the Hulk who has lost an arm, much like his MCU counterpart. We know using the Mind Stone can corrupt someone mentally. Maybe using the Infinity Stones here could influence the Hulk down the line, driving his frustrations with the world to a global scale.
The MCU will need new threats going forward, and seeing one of the most powerful heroes in the MCU (or at least a version of him from a potential future) turning on the heroes could make for a worthwhile threat. While the Hulk not getting his dues following Avengers: Endgame might seem like a petty reason to turn the Hulk evil, it could be the first part of a slippery slope that leads to someone like Maestro coming to the MCU.