As all Marvel fans know, Robert Downey Jr is contracted for two more Avengers movies, Avengers 2: Age Of Ultron and [The Avengers 3](movie:738027) - but after his enigmatic comments at this year's San Diego Comic Con, many are speculating that his final appearance may actually be in Iron Man 4.
So if we assume this is the the case and Robert Downey Jr makes his final appearance as Marvel's most iconic character, the plot would need to centre on his stepping down as Iron Man: but what could make Tony Stark take such a drastic move?
Let's outline some of the possible options, but beware - after this come potential Marvel:
Okay, so one iconic storyline that Marvel must have in the pipeline at some point in their Cinematic Universe is the infamous Avengers: Civil War.
The Avengers Civil War: The End of Iron Man?
This is one of the most complex, all-encompassing story lines in the entire Marvel universe. It begins with the introduction of the Superhuman Registration Act.
After years of Superhero mayhem the public have grown tired of the destroyed buildings, death and fear that comes with superheroes battling each others all across their towns and cities. The Superhero Registration Act is one proposed solution, allowing the government to monitor their super powered citizens and, if necessary, to reign them in.
It is a controversial proposal in the superhero community, but one backed by SHIELD, Ant-Man and Tony Stark, who by now has a senior position in the organization.
In the other camp, however, is Captain America. Captain America has always stood for the freedom of the individual, and against big, controlling governments. This, of course, comes from his experience of fighting the Nazis in 1940s Germany - and it is something that has already been hinted at in the Marvel Cinematic Universe. Do you remember this interaction with Nick Fury in [Captain America: The Winter Soldier](movie:254973)?
Nick Fury: (SHIELD is) "going to neutralize a lot of threats before they even happen,"
Captain America: I thought the punishment usually came after the crime.... This isn't freedom. This is fear."
To cut a long story short, the two camps become so entrenched against each other that it turns into a bloody and disastrous civil war, with bitter attacks, retaliations and counter-retaliations on each side.
Finally, after Iron Man and SHIELD have built a prison to house all non-complying superheroes, there is a final battle in which Captain America surrenders. He is taken to court, and on the steps of the courthouse, he is shot and killed.
Naturally, Tony Stark is distraught at the loss of his former friend. In a state of bitter despair, he resigns his title as Iron Man:
Of course, there are other options for Robert Downey Jr's departure from Iron Man. Fans have often called for the iconic Demon in a Bottle storyline, which sees Tony Stark succumb to alcoholism, could this be the reason for his stepping down?
Alternatively, might he simply decide to disappear into Space, perhaps meeting up with Marvels latest stars in [Guardians of the Galaxy 2](movie:1081113)?
Whatever the exit strategy for Robert Downey Jr in Iron Man 4, we're sure it'll be an incredible journey.
But what do you think is the most likely reason Marvel will come up with for Robert Downey Jr's departure in Iron Man 4? Have your say in the comments sections below!