Joe and Anthony Russo have come together once more to tackle an impending Marvel Cinematic Universe release, this time dropping what looks to be the biggest superhero movie of all time with 'Avengers: Infinity War'. Bringing crime fighters both old and new to the fight against Thanos (Josh Brolin), the roster of characters in 'Infinity War' will break all records of the genre.

Robert Downey Jr as Tony Stark in 'Avengers: Age of Ultron'Robert Downey Jr as Tony Stark in 'Avengers: Age of Ultron'

Exactly what's going to go down in 'Infinity War' remains to be seen, but we have been told as a fan base to expect casualties and upset. Could it be possible that one of the leading stars of the MCU is headed out of the door, and if so, is Robert Downey Jr's future as Iron Man in danger? We doubt it just yet, as he's signed on for the currently untitled fourth 'Avengers' movie, but a curve ball wouldn't come as too much of a surprise.

In Marvel's comic books, the Iron Man mantle has been taken on by different people, but that doesn't seem to be something Joe Russo is interested in bringing over to the movies.

According to the Hindustan Times, the co-director said that "there is nobody that can take over Iron Man from Robert", but admitted there would be a time when the actor left the role behind.

He explained: "He can’t play the character forever. I am sure there are lots other things he wants to do in his career. But he loves playing the character, I know that, and we love working together. We will see where it goes from here."

For now, we'll just have to sit back and enjoy every bit of Downey Jr we can get in the role. He's certainly cemented a legacy as Iron Man, and so whenever he does choose to give it up, it's something he'll be forever remembered for. Russo is quite right in that he's irreplaceable.

More: Joe Russo Promises 'Avengers 4' Will Finish 'Winter Soldier' Story

'Avengers: Infinity War' is set to come to cinemas across the UK on April 26, 2018.