Despite all of the homages to the story in Thor Ragnarok, I still feel like Planet Hulk has to happen at some point.
In fact, it would be a great way to kick off Phase 4, if they're going to follow the same structure as Phase 1-3. Something happens in Avengers 4 that causes the "Illuminati" to make the decision to exile Hulk. Then the Planet Hulk story plays out, leading to Avengers 5: World War Hulk.
I feel like the World War Hulk story HAS to happen considering just how much they've been setting up Hulk as a threat across all of these films. They also haven't shied away from Tony being a total asshole/borderline villain, even to the other Avengers, which plays a big role in the Planet Hulk/WWH storyline, of course.
Like Avengers 1, you could have World War Hulk happen, and the introduce the REAL big bad of the next phases of movies either at the end or in the post credits. Maybe by then Marvel has Doom or Galactus back. Or they choose someone else entirely.