Mark Ruffalo’s iteration of the Hulk and his science genius counterpart Bruce Banner has steadily evolved over a decade of interconnected movies. First introduced in The Avengers, Banner went from struggling to control his alter ego’s hot-headed tendencies to tempering them completely in Avengers: Endgame. However, this transition from angry to calm Hulk may unravel in Phase 4 of the Marvel Cinematic Universe, potentially paving the way for an instrumental Hulk comic book storyline to emerge in Avengers 5.
In the original comics, an experiment meant to recreate the super-soldier serum — the very same experiment Captain America underwent during World War II — exposed Bruce Banner to gamma radiation. Instead of simply being endowed with immense strength and agility, Banner transformed into the raging green creature known as the Hulk.
In the MCU, Banner was initially reluctant to join the Avengers, choosing to hide out of embarrassment and an inability to control his Hulk-like tendencies. Even after joining Earth's mightiest heroes, he was never able to get a handle on his alter ego.
That, in turn, prevented him from properly developing both sides of himself. Thor: Ragnarok saw Banner’s evolution regress. Stuck in Hulk’s form for two years, his rage went unchecked. Meanwhile, the gladiatorial fights on the planet Sakaar provided a meager outlet for the Hulk's temper. Banner’s struggle to take the driver’s seat continued through to Infinity War when the Hulk refused to emerge during the climactic battle with Thanos.
In the five years after Thanos snapped half of the world’s population out of existence, Banner and Hulk peacefully merged together to create an amalgamation of the two personas. Endgame explained that Hulk wasn’t a disease, but the cure. And so, Banner locked himself in his lab and worked on putting the “brains and the brawn together” to create the “best of both worlds.”
But Endgame’s explanation of “Smart Hulk” leaves out a great deal. It’s unclear whether this merge was actually something Hulk wanted or if Banner forced it (Hulk's personality is seemingly gone, while his body remains). What’s more, there’s no telling whether the Smart Hulk persona will stick or if something will eventually cause Banner and Hulk to become divided once more.
Will Smart Hulk be the one fighting in Avengers 5?
Enter, Planet Hulk. In the comics, this storyline, which ran for 14 issues of Incredible Hulk from 2006-2007 and led into the World War Hulk comics crossover, saw Hulk effectively lose control of his rage after the detonation of a gamma bomb. The Illuminati — whose members include Iron Man, Doctor Strange, Fantastic Four’s Reed Richards, and X-Men’s Professor Xavier — deem Hulk too unstable and threatening to remain on Earth and trick him into a space mission that ultimately sends him to Sakaar.
While there, he’s enslaved and forced to fight gladiator battles. Long story short, Hulk helps the rebels defeat the Red King, Sakaar’s warlord emperor. But the story takes a tragic turn when the shuttle that brought Hulk to Sakaar explodes and kills his wife, Caiera.
Blaming the Avengers, Hulk heads back to Earth for revenge, setting up World War Hulk storyline. Basically, it was Hulk vs. everyone else. Thor: Ragnarok already adapted parts of the Planet Hulk storyline (namely the Sakaar gladiator stuff), but it’s possible that Phase Four of the MCU may explore more of Hulk’s out-of-control nature in that context.
Banner ended Endgame with some physical and emotional scars. It's easy to imagine "Smart Hulk" becoming unhinged as Phase 4 goes on, climaxing in a battle between Hulk and everyone else in Avengers 5. Either that, or a gamma bomb will go off and force Hulk to revert to his usual smash-happy self.
It would be fascinating to see Hulk as an antagonist, even if Avengers 5 doesn't play out exactly like the Planet Hulk storyline. We want a bit more of that Hulk smash energy before the character skulks away for good.