Here's a weird twisteroo: Marvel doesn't have the rights to make an Incredible Hulk standalone movie. Those rights remain at Universal, the studio that released the Edward Norton version. And this isn't hearsay, it comes directly from Mark Ruffalo:
As far as a Hulk movie, a standalone Hulk movie, Marvel doesn’t really have the rights to that yet. That’s still Universal’s property, so there’s that issue. That’s a big impediment to moving forward with that. Now I don’t think that’s insurmountable, by the way, but I don’t know where it’s going from here for me.
They own Hulk, but Universal still has a deal to release his standalone movies. Obviously if they so desired Disney could simply make a deal - they made a deal to take Avengers and Iron Man Three off Paramount's hands back in the day. They would probably give Universal an upfront amount and a small cut of the gross, as they did with Paramount. But right now it seems like they don't think it's worth the effort.
All of this makes a lot of other rumors make much more sense. What follows are spoilers for Hulk's final scenes in Avengers: Age of Ultron (although spoilers that have been reported for quite some time):
Early drafts of the script had Hulk being rocketed out to space. The final film is more circumspect - Hulk is in a Quinjet very, very high above the Earth, but not quite out of the atmosphere. I had heard that Marvel was considering sending Hulk into Guardians of the Galaxy 2, especially if the first film didn't hit - having a Hulk in the mix could goose the box office on the sequel. But with this knowledge that plan makes even more sense - since they couldn't make a standalone Planet Hulk movie, they would use Guardians 2 to back door it!
Of course Guardians was a major hit and they don't need to goose the box office for the sequel, so the Hulk is sort of up in the air. Ruffalo himself doesn't know when he's back:
Sometimes these things happen at the last minute in this world. I really don’t know where I fit in from here on out, and I’m not sure they do.
I'm not even sure Marvel has decided where Hulk even is right now. Talking to IGN, Ruffalo said:
Well he's going to space. Yeah, I mean there's some talk of maybe Planet Hulk.
Which he walked back in a later interview - but that first sentence makes me think Ruffalo believes the original ending of the script is still in play. And again, the ending of the movie is vague enough that Marvel could make any decision moving forward. The studio has plenty of plans, but they're always very fluid in how they approach them. Things change on a dime, so what's true today - like Hulk in space - may not be true tomorrow.