The last time we saw the Hulk, he was soaring away from Black Widow in a Quinjet in Avengers: Age of Ultron. Between then and now, not much has been revealed about his whereabouts and how it'll fit into Phase 3 as the Avengers gear up to face the impending doom that Thanos will bring in Avengers: Infinity War.
There were rumors that Mark Ruffalo was set to co-star alongside Chris Evans and Robert Downey Jr., among a roster of others, in Captain America: Civil War. However, these rumors were soon debunked by the man behind the Green Avenger.
Now, in a rumor that movie website JoBlo claims to be from inside sources, it appears that our answer to the Hulk's whereabouts may be explored in Thor's upcoming third film, Thor: Ragnarok.
"Hot on the heels of NYCC [New York Comic Con], we’ve gotten word from our inside sources that THOR: RAGNAROK will see Chris Hemsworth’s Thor team up with none other than Mark Ruffalo’s HULK for the Odinson's third solo film, which finds the God of Thunder facing what amounts to the Norse Apocalypse. Only this time out he'll have some help from a fellow Avenger."
The article also claims that Thor's third film will explore a planet that is neither Asgard nor Earth. This further leads to the idea that Marvel may be weaving a Planet Hulk storyline into the narrative of the Marvel Cinematic Universe. However, if the rumor returns true, one may wonder how it will play into Thor's final solo feature.
In the Planet Hulk storyline, the Hulk is deemed too dangerous to remain on Earth. When he is sent on a mission to destroy a satellite by the Illuminati (a behind-the-scenes group of superheroes including Iron Man, Doctor Strange, Mr. Fantastic, and Professor X in the Marvel Comics universe), it is intended that he be sent to an undisturbed planet. However, the Hulk finds himself passing through a wormhole and landing on the planet Sakaar, where he is enslaved and forced to be a gladiator for the Red King, the emperor of the planet.
How this will be incorporated into Phase 3 is anyone's guess. Since Marvel doesn't have the distribution rights to the Fantastic Four or the X-Men, use of the Illuminati is rather slim, leaving it to the Hulk to either a.) travel into outer space on his own and end up on Sakaar, or b.) have some force catapult him into that storyline. Either of those options are plausible if this is the direction that Marvel is heading in. But then the question of how Thor comes across this new development arises.
At the end of the day, it is a possibility that the Hulk will appear in Thor: Ragnarok if his whereabouts are not tackled in a movie prior to the film's release. And if some story elements are used from Planet Hulk, it should prove an exciting adventure worth taking as Marvel drifts slowly from Earth-level stories and moves further into the cosmic.
Thor: Ragnarok is slated for a November 3, 2017 release.