In February of 2013, El Mayimbe of Latino Review reported that Marvel was planning to launch Hulk into space at the end of the Avengers sequel, which would lead to a Planet Hulk film. Since then, there have been many denials. Most people have put that rumor to bed. Then a year later something odd happened - Thor: The Dark World concept art revealed that Marvel commissioned concept art for the Oldstrong and Kaifi. They're two characters that first appeared in Marvel's "Planet Hulk" comic book storyline. Those characters did not appear in Thor 2, but at one point they were considered.
Now, Kevin Feige is out promoting Guardians of the Galaxy and IGN asked him about the chances of a Planet Hulk film.
"I think what's cool about the Planet Hulk story is it's all Hulk. He's a gladiator, he's fighting all these creatures on another world. Yes, some of those creatures show up in [Guardians of the Galaxy], which makes people think, 'Oh, maybe it could exist.' And maybe it could exist, someday. I don't think that's the first place we would go though for the very reason that you stated, which is, if we were ever able to do a standalone Hulk movie, 60 percent of the fun would be seeing Mark Ruffalo leading a Marvel movie -- and that's what we would do if we ever went down that road." - Kevin Feige
It's worth noting that, while Feige said there were no immediate plans to tackle Planet Hulk, he did not entirely rule out the idea of "Hulk in space" or a Banner/Hulk-led film, meaning that may still be on the table in some fashion.