"That was Kevin," he told Omelete. "Interestingly enough about that, that conversation was had a while ago."

"Robert [Downey Jr] tried to talk us into doing it and we said, 'No.' We just didn't have a story. We didn't have a way in, right? We were resistant for a while."

"And then one day [Endgame writer] Steve McFeely, who's one of our key collaborators, said, 'I have an idea.' And we went, 'That's the story. That story has to be told. It's a really powerful story.'"

There is very little information on Avengers: Doomsday available except that Downey is playing the titular villain, and the Russos intend to give nothing else away.

"The one thing I will say about the movie is this: we love villains who think they're the heroes of their own stories. That's when they become three-dimensional and when they become more interesting. And when you have an actor like Robert Downey, you have to create a well-shaped three-dimensional character for the audience," Joe Russo said.

Avengers: Doomsday is set to hit theatres in 2026, while the Fantastic Four reboot hits theatres this July.

The Fantastic Four: First Steps stars Pedro Pascal, Vanessa Kirby, Ebon Moss-Bachrach, and Joseph Quinn as the heroic quartet, with British actor Ralph Ineson as the villain Galactus and Julia Garner as the Silver Surfer.

At the time of Downey's casting announcement, the Russo brothers said it was made possible due to the multiverse - a series of different worlds featuring characters old and new, which makes unlimited possibilities available.

This suggests that Downey will be playing a villainous variant of Tony Stark/Iron Man. It's believed this idea was thought up after the axing of Jonathan Majors from the MCU; his character Kang the Destroyer was originally meant to be the primary villain in a forthcoming Avengers film.