Thor: Ragnarok is the first part in a larger Hulk story, according to actor Mark Ruffalo. Based loosely in the Incredible Hulk comic book arc "Planet Hulk," Thor: Ragnarok features Ruffalo as Hulk alongside the titular Chris Hemsworth.
"So basically, Kevin [Feige] pulled me aside before this, and said, 'If you were gonna do a... if we were going to do a standalone Hulk movie, what would it be?' And I said, 'I think it should be this, this, this, and this and this, and ends up like this,'" Ruffalo told CinemaBlend. "And he's like, 'I love that. Why don't we do that in the next three movies, starting with Thor 3 and then we go into Avengers 3 and 4.' And I was like, 'That sounds great!' And so we are at the beginning of this arc."
That would be November's Thor: Ragnarok, May 2018's Avengers: Infinity War, and May 2019's untitled fourth Avengers film.
So what is this larger Hulk arc about? Ruffalo answers, saying "Hulk speaking is the start to separating of these two individuals, these identities in this split-identity person, and where that's going to end up going. So it was really interesting to me. He's like a baby! He's like a five-year-old or six-year-old. So he has the same syntax, he has the same world view, and so it was fun. It's like Chris - we all got to reinvent our characters in this."
Ruffalo has previously lamented the inability for Marvel Studios and Universal Pictures (which own the movie rights to Hulk) to come to terms to do a new Hulk film. The two studios worked together for 2008's MCU tie-in Incredible Hulk, but since then have not been able to come to terms for another with Ruffalo - who took over the role beginning with 2012's Avengers.
"I wanna make just one thing perfectly clear today, a standalone Hulk movie will never happen," Ruffalo said in July. ".... for some reason they don't know how to play well with Marvel."