In the Marvel cinematic universe, Marvel left the fate of Hulk (Mark Ruffalo) up in the air at the end of Avengers: Age of Ultron. In that film, the last scene of Hulk showed him on board a jet that crashed into the ocean. After that, there was no sign of him and the Avengers thought him lost.
Of course, it's not likely that Hulk died in that crash, but since Age of Ultron, Marvel hasn't said much on when we might actually see the character again.
However, JoBlo reports that its sources at Marvel have confirmed that Hulk will appear back in the Marvel cinematic universe soon, possibly even before fans might think. Although it's likely that fans will get to see Hulk in Avengers: Infinity War in 2018, JoBlo reports that Hulk will show up even earlier than that, in 2017's third Thor movie titled Ragnarok.
Although Marvel hasn't confirmed this news publicly, JoBlo reports that Hulk will join Thor in Ragnarok and help him fight off an apocalypse of Norse proportions. The site also reports that much of the setting of the third Thor film will happen on a "distant planet" that is "not Asgard and not Earth."
Of course, this leads to speculation about a potential Planet Hulk storyline as seen in the Marvel comic books. In that story arc, the Avengers send Hulk away and he ends up on his own planet where he eventually becomes leader and then decides to take out his revenge on Earth for being so mean to him.
Of course, Marvel has also not confirmed a Planet Hulk storyline, but if Hulk's appearance in Thor 3 gets confirmed and the storyline is really about a distant planet, it's a definite possibility that Planet Hulk could play into that film somehow. Marvel, however, has consistently denied that this is a storyline it wishes to pursue, but the company generally remains secretive on the plots of its future films.
However, Marvel seems to have jumped on the idea of teaming two or more Avengers for its future solo movies, much like it did with Captain America: Winter Soldier: that film not only brought back Black Widow, but also introduced Falcon. An Ant-Man and Wasp duo film is also now in the works.
JoBlo also reports that Tom Hiddleston will return to Thor 3 as Loki: fans haven't seen that character since the second Thor film. Also reportedly returning to the Thor franchise is Jaimie Alexander as Sif.