We recently heard more casting for Thor Ragnarok including both Jeff Goldblum and Karl Urban being added to the cast. Now, Joblo.com is claiming to have some knowledge of the plot of the film. Their sources say that the film will include elements of Planet Hulk, the Greg Pak storyline that dropped the gamma irradiated monster into gladiatorial games. Many fans have been calling for Marvel to adapt this film into live-action (as it has been done for animation).
The third Thor film will combine elements of Ragnarok with Planet Hulk where Thor (Chris Hemsworth) ends up on a planet where Hulk (Mark Ruffalo) is competing in gladiator tournaments hosted by the Grandmaster (Goldblum). Word is that Hulk will be wearing armor similar to his appearance int he comic series. Thor will look different as well, with the Thunder God losing his long hair for a portion of the film.
As for why Thor is on the planet, he is looking for a weapon that will set the world right again and prevent Ragnarok. Odds are the weapon is somehow tied to one of the two missing Infinity Stones.
Its interesting to see the Grandmaster being brought in here in that he is a member of the Elders of the Universe along with The Collector who was introduced at the end of Thor: The Dark World before appearing in Guardians of the Galaxy. The original Contest of Champions was set up as a wager between the two.