The appearance of the Hulkbuster armor has sparked speculation about the plotline of Iron Man 4. One of the more intriguing story arcs is Planet Hulk. It could be a good take-off point for a Hulk solo movie.

All the elements are falling into place. First, the fall of SHIELD brought more power and responsibility to Tony Stark. JoBlo shared this spoiler: |Tony Stark is indeed bankrolling The Avengers this time out. With the dissolution of S.H.I.E.L.D. in CAPTAIN AMERICA: THE WINTER SOLDIER and it's subsequent "handover" to Phil Coulson in Agents of S.H.I.E.L.D., Tony Stark takes on the role of global security by housing The Avengers in the former Stark Tower, now Avengers Tower."

It is this new duty that prompted him to take extreme measures. The second factor is the Hulk getting out of control.
From Screenrant: "Hulk's character in The Avengers 2 - a film which Whedon says will take the characters to a dark place - will hit an all new low when it comes to losing control and it'll force Earth's Mightiest and the Marvel cinematic universe's version of the Illuminati (secret council of high-profile superheroes) to launch the big green giant into space where he can do no harm. Fans of the books know what this means and yes, Planet Hulk is what these actions result in."

In the tradition of not letting the rumor mill win, this has already been refuted. Joss Whedon spoke to MTV:

"I don't think there's a lot that we couldn't do someday, as the cinematic universe continues to grow and expand and get as big as the comic book universe. 'Planet Hulk' is a cool story. 'World War Hulk' is a cool story... I think there are pitfalls of continuity-overload, and mythology getting so dense that it almost collapses in on itself. It happens every few decades or so in the comics. Apart from that, I'd say everything is on the table."

"Do I think Hulk can carry a movie and be as entertaining as he was in 'Avengers'? I do believe that. I do believe he absolutely could. We certainly are not even going to attempt that until 'Avengers 2. So there's a lot of time to think about it."

But Tony Stark has already been wary of the Hulk. He made the Hulkbuster armor specifically for that. It may sound like betrayal, but this is how Marvel presented the dilemma of the superhero, especially when you take major responsibility.

'Iron Man 4' can introduce the new possibly post-Thanos phase in the same way 'Iron Man' started the ball rolling in 2008. It is hoped that Robert Downey, Jr. will be stay for the ride.