The Avengers' resident big green bruiser has been through a lot. He's hated himself, found love, lost love, spent long periods trapped in both of his forms and even learned to coexist with his dual personalities. What is there left to do for Hulk in the Marvel Cinematic Universe? He certainly didn't get as definitive an end to his journey in Avengers: Endgame as Iron Man, Captain America or Black Widow, so it leaves him in an odd position.
While most people will move on and continue to follow the example led by the original team, what can Professor Hulk do Following the events of Avengers: Endgame -- where he came to believe that he was destined to snap the missing half of life in the galaxy back into existence by using Stark's Infinity Gauntlet -- Banner looks to still be injured, and it is undetermined how long this will keep him sidelined.
It should also be taken into account that Bruce might not want to return to active duty as an Avenger. After much apparent soul-searching that led him to become Professor Hulk, he shows a visible distaste for violence and the brawny theatrics of the Hulk from yesteryear. This could mean he's poised for more of a support role in the superhero organization, consulting where necessary.
With Spider-Man: Far From Home continuing the exploits of young Peter Parker, one might wonder, if not Bruce Banner, who will be his powerhouse counterpart going forward. The MCU may need a new Hulk in the form of an Amadeus Cho or perhaps She-Hulk; someone who could continue Hulk's legacy without Banner's direct involvement.
GOING OUT ON A LIMB
Unlike some of his original Avengers counterparts, Hulk wasn't seen moving on following Endgame. He was still very much involved, helping Steve, Sam and Bucky in their post-conflict tasks. It wouldn't be unreasonable to assume that Hulk is happy to continue sharing his knowledge to help his newer friends, but can he still go like he used to? He expressed that he's moved on considerably from just smashing up his problems when he traveled back to the battle of New York and was presented with his past self. Perhaps he no longer has the stomach for that level of destruction anymore.
If we examine his role in the final third of Endgame, we see Hulk coming to terms with the fact he should be the one to resolve the snap with his own. He accepts it as his destiny, correlating the infinity stone's gamma radiation to his own body and origin. The snap to bring back those lost physically damaged Hulk's arm to the point where he could no longer use it.
This is alarming, as previously the Hulk has been shown to be able to recover from an amazing amount of physical trauma. The extent of Hulk's healing in the MCU has never been explicitly stated. However, when he can survive shooting himself and heal from having his skin pierced by the teeth of a mythical Asgardian wolf, it is alarming that his arm remains out of action, even in scenes some time after his snap.
THE NEXT STAGE OF RAGE
If Bruce is indeed out of commission in terms of being out in the field, the world and the Avengers may need a new physical powerhouse. The most immediate suggestion for a replacement would be fan favorite and relative to Banner, She-Hulk. Jennifer Walters would be a great addition to the Avengers and the MCU as a whole.
We've already seen some glimpses of battles in courtrooms in Marvel's Netflix series' and we know the MCU knows how to frame hard-hitting action. These two aspects make up the DNA of She-Hulk, a dedicated practitioner of law and a tank in battle. She would also be a good focal point for an A-Force assembly, first nodded to in the final battle of Avengers: Endgame.
If not Jennifer, the next replacement could be Amadeus Cho, who was the Totally Awesome Hulk for a while. During his time filling in for Bruce in the comics, he had an interesting run of stories where he would pursue large, dangerous creatures and detain them to prevent them from destroying civilian areas.
This could be quite the spectacle if brought to the big screen and it would establish his physical prowess, as well as his genius-level intellect as he adapts to each threat. Not only this, but Cho is a younger hero, lending himself to a role in the future of the MCU and an ideal torch-bearer for Banner's legacy.
There are other options to consider, also. Rick Jones, Bruce Banner's best friend in the source material, has been referenced as far back as Avengers Assemble. Knowing that he exists in the MCU, he could possibly make an appearance, taking over from his friend as his own Hulk-like alter ego from canon, A-Bomb. With Thor seemingly leaving to join the Guardians of the Galaxy, the MCU might see the introduction of Hercules, who would not only fill the void for the missing God, but mirror the time he became the Incredible Hercules, taking over from Hulk for a short time with the help of Amadeus Cho.
Worst case scenario? Thunderbolt Ross decides to become Red Hulk, and given his past tension with the Avengers, it will not end well!