One long-running MCU mystery is finally addressed in a new Captain America: Brave New World theory, explaining how the Red Hulk debuts in the upcoming Phase 5 movie. The first teaser trailer for Captain America: Brave New World, released on July 12, 2024, revealed a lot more about Anthony Mackie’s big-screen debut as the MCU’s new Star-Spangled Man-with-a-Plan. Brave New World will not only pit Sam Wilson’s Captain America against Tim Blake Nelson’s Leader and Giancarlo Esposito’s mysterious gun-toting villain, but will also bring him to blows with the Red Hulk.

The introduction of the Red Hulk in the MCU has been heavily speculated for a number of years, and many were excited to see the late William Hurt’s General Ross eventually evolve into the brutal Marvel supervillain. Unfortunately, Hurt’s tragic passing in 2022 made this impossible, but Harrison Ford was recast as Thaddeus “Thunderbolt” Ross, and is set to make his debut as the new President of the United States in Captain America: Brave New World. Ross being recast means the Red Hulk can finally show up, and a new MCU theory may explain exactly how this happens.

President Ross Becomes Red Hulk In Captain America: Brave New World

Thaddeus Ross’ return as the United States’ President in Brave New World is the next logical step for his character, who previously rose through the government’s ranks as General and, later, Secretary of State. However, it seems he will also be making his MCU return as Red Hulk, given the fact that it’s General Ross who was transformed into Red Hulk in Marvel Comics’ Hulk (Vol. 2) #1 in 2008. Since Red Hulk only appears briefly in Captain America: Brave New World’s trailer, it’s unclear how this transformation happens, but a new theory might have the answer.

President Ross Has Been Enhanced By Extremis: MCU Theory Explained

A game-changing MCU theory posed by Reddit user BradmanTV suggests that President Ross may become Red Hulk in Captain America: Brave New World because of Extremis. First seen in 2013’s Iron Man 3, Extremis was created by Aldrich Killian and Maya Hansen as a means of regenerating the human body, but the substance was incredibly volatile. The theory posits that a refined version of Extremis was used on Thaddeus Ross during his major surgery following a heart attack, which he mentioned in conversation with the Avengers in Captain America: Civil War.

Thaddeus Ross has always been a huge asset to the United States government, especially given his work trying to restrict the actions of unpredictable superheroes. This means it is entirely plausible that a refined, safe version of Extremis would be used on him, should that save his life. This Extremis could perhaps be lying dormant in his system following his surgery, but the theory suggests that something such as Marvel Comics’ Gamma Bomb could merge with this Extremis and transform Ross into the Red Hulk.

It’s possible that Samuel Sterns’ Leader could detonate a Gamma Bomb in Captain America: Brave New World, which has the potential to create even more Hulk-like characters, including the Red Hulk. This theory perfectly explains why and how President Ross becomes Red Hulk, and connects his evolution into this iconic Marvel villain to several projects in the MCU’s history. Most notably, this would address the mystery of Extremis, which has barely been seen or mentioned since Iron Man 3​​​​​​​.

Extremis Has Only Been Seen Twice Since 2013’s Iron Man 3

Extremis was set up as a major element of the MCU in Iron Man 3, but the threat posed by the dangerous substance was dealt with by the end of the Phase 2 movie, and was barely touched on again. Extremis did make its first reappearance in the MCU in 2021’s Shang-Chi and the Legend of the Ten Rings, as one of the fights in Xu Xialing’s Golden Daggers Club was enhanced with the substance. This was perhaps included to remind viewers that Extremis existed before it had a slightly bigger role in 2023’s divisive Secret Invasion​​​​​​​.

Secret Invasion saw Kingsley Ben-Adir’s Skrull Gravik enhance himself with several new abilities, stolen from past MCU projects. As well as enhancing his strength and being able to grow his limbs, Gravik’s Super Skrull experiments also infused him with Extremis. This allowed him to regenerate after being stabbed by Talos and shot by Nick Fury, and presumably granted him the heat-based capabilities of Extremis, too. Extremis deserves to have a bigger role in the MCU again, and Captain America: Brave New World can make this happen.

Emilia Clarke’s G’iah also enhanced herself with Extremis, as well as the DNA of other significant characters from the Harvest, becoming the second and, after Gravik’s death, only Super Skrull in the MCU.

Marvel Comics’ Red Hulk Shares Many Abilities With The MCU’s Extremis

Although not exactly the same, the capabilities of Extremis in the MCU and the power-set of Red Hulk in Marvel Comics are remarkably similar. This means it wouldn’t be too much of a jump to explain Red Hulk’s unusual powers as being the cause of Extremis in Captain America: Brave New World. While Marvel Comics’ Red Hulk has many of the same abilities as Bruce Banner’s Hulk, he also emits huge amounts of gamma radiation that produces heat and often makes him look on fire, which reflects Extremis perfectly.

In the MCU, as well as receiving a fast regenerative healing factor, Extremis soldiers are able to generate an immense amount of thermal energy. They can control the temperature and intensity of this energy at will, up to thousands of degrees Celsius. Trained Extremis soldiers can emit this heat from focused areas of their bodies, such as their hands or arms – Aldrich Killian even breathes fire. Transforming President Ross into Red Hulk with Extremis would grant him these impressive gifts, which could create one of the MCU’s most dangerous supervillains ever.

How Else Could Extremis Be Important To Captain America: Brave New World’s Storyline?

As well as dealing with the emergence of the Red Hulk, Captain America: Brave New World will also be putting the focus back on the MCU’s super-soldiers. Most notably, the first teaser trailer for Brave New World revealed that super-soldier Isaiah Bradley will be manipulated into making an assassination attempt on President Ross. This could prompt extreme action by the United States government, which could perhaps revive one of Agents of SHIELD’s old Extremis-focused storylines by bringing back the Centipede Project.

For the Centipede Project, HYDRA combined Extremis with gamma radiation and Chitauri technology to enhance individuals into super-soldiers. This storyline could be reinvented for Captain America: Brave New World, as the government may want to create its own army of super-soldiers to counteract the rise of more superhumans in the MCU. This means Captain America: Brave New World could feature an epic return to form for Extremis, continuing a story over a decade in the making.