In a recent interview with Digital Spy, Mark Ruffalo talked about the possibility of a Hulk standalone movie.
"I think they are, for the first time, entertaining the idea of it. When we did The Avengers it was basically 'No!', and now there is some consideration for it."
A few things to chew on: Ruffalo has a six movie deal with Marvel. Over a year ago rumors of Planet Hulk being adapted into a movie surfaced. Since then, talk about it fizzled out, until copies for the script of Avengers: Age of Ultron started circulating. The newest rumor is that the latter third of it helps setup Planet Hulk. I've included the trailer to the animated Planet Hulk feature if you are not familiar with the graphic novel.
When you watch the video of the interview, look at Ruffalo's body language when he says:
"But there's still nothing definitive, not even a skeletal version of what it would be. I look forward to going down that road, if we could crack that nut."
I don't want to straight out call him a liar, but if this was poker, that would be huge "tell." I think they cracked how to do a Hulk film over a year ago and are just waiting for the right time to do it.