WARNING: The following article contains major spoilers for Avengers: Endgame, in theaters now.
Avengers: Endgame introduced a whole plethora of classic comic tropes and potential future directions for the Marvel Cinematic Universe to embark on following the conclusion of its Infinity Saga. In addition to introducing the Multiverse through the reality-altering effects of the the Infinity Gauntlet and divergent timelines via time travel through the Quantum Realm, the film may have set up the next major MCU villain... and it could be an alternate version of a founding member of the Avengers.
Bruce Banner had successfully merged his own personality with his gamma-radiated alter ego to form a new one dubbed Professor Hulk. Combining Banner's scientific intelligence with the Hulk's incredible strength, the Avenger used a version of the Infinity Gauntlet to restore half of all life in the universe albeit resulting in permanent damage to his right arm. And, despite possessing ultimate power with the Gauntlet, Banner was unable to resurrect his longtime love interest and teammate Black Widow due to her connection to the Soul Stone. These losses weighed heavily on Bruce and, as the timelines begin to diverge, could signal an alternate future Professor Hulk returning as a classic Marvel Comics villain: The Maestro.
Created by Peter David and George Perez in 1992's The Incredible Hulk: Future Imperfect #1, the Maestro came from an alternate, post-apocalyptic future where nuclear war had wiped out all of the Earth's superheroes and most of the human race. The Hulk not only survives but grows significantly more powerful by absorbing all the nuclear radiation, augmenting his already staggering strength and abilities. The mass absorption of radiation coupled with decades of resentment for his treatment by the world has the unintended side effect of driving the Hulk insane. Retaining Banner's intellect, the Maestro brings the remnants of humanity under his dictatorial rule, with rebels using a time machine to bring Professor Hulk from the past to save them.
With divergent timelines now firmly part of the MCU, one possible future could involve a version of Professor Hulk transforming into the Maestro and somehow returning to haunt the main timeline and Bruce Banner as the worst possible outcome of his new merged personality realized. And it could all stem from Professor Hulk's taste of omnipotent power while briefly wielding the Infinity Gauntlet.
As the old saying goes, absolute power corrupts absolutely and the Infinity Stones are no exception. Marvel Studios had confirmed that Loki's exposure to the Mind Stone during The Avengers actually brought out his darker, more villainous tendencies beyond his control and the other Infinity Stones have been shown to have varying degrees of corrupting effect. In briefly tasting ultimate power and the revelation that the Infinity Stones still exist on an atomic level, could an obsessive parallel reality Banner accidentally trigger an apocalypse that would transform him into Maestro?
While ostensibly from another reality, travel between divergent timelines and parallel universes has been teased as possible within the MCU before. Mysterio and the Elementals from Spider-Man: Far From Home claim to be from an alternate Earth while Captain America's reappearance in the main MCU timeline after creating a divergence decades in the past suggests this is possible, though the particulars have yet to be revealed. And with these dimensional rifts, the Maestro could extend his plans into the main MCU timeline in an effort to change history or simply spread his rule.
The Hulk has always been one of the most unpredictable superheroes in the Marvel Universe due to his complete lack of inhibition and constant resentment for how he's treated by the world. Still guilt-ridden over being unable to resurrect Natasha Romanov and having tasted the infinite, if only for a moment, Banner's usual obsessive nature could lead a version of the character to take a surprise villainous turn, twisted further by the radiation left after three coscmic, reality-changing snaps. If this alternate timeline Hulk is anything like the Maestro, the heroes of the MCU will have to face one of the most cunning and physically strongest threats capable of smashing the whole multiverse.