The latest issue of Total Film magazine has a featured article on Thor: Ragnarok where both Hemsworth and director Taika Waititi confirm elements from Planet Hulk.
"I was talking with the guys at Marvel, even before Taika came on," Hemsworth said. "It was about: 'How can we show something of this world that people haven't seen?'"
As the article notes, the answer is "a trip to a planet that's all kind of weird."
"Sakaar was just an insane, far-off futuristic place which was the biggest shift for the film and these characters," director Taika Waititi said.
The article further notes that "Sakaar is also where Thor will reunite with Mark Ruffalo's Hulk, who has made a name for himself as a gladiator in a plot arc mirroring acclaimed comic series Planet Hulk."
While it remains to be seen how Marvel Studios will unfold the tale, the comic book series of Planet Hulk sees the Hulk forced off the Earth by the Marvel heroes and mistakenly sent through a wormhole where he ends up on the planet Sakaar and fights in a gladiator arena.
From the above, we can speculate something happens to the Hulk following The Avengers: Age Of Ultron that causes the Hulk to end up on Sakaar and become a gladiator. The info also makes it apparent that the Hulk has been on Sakaar for some time as he has built a name for himself, and then Hulk eventually meets up with Thor.
It's also thought we may see the Hulk's Warbound in the film as both Korg and Miehk have been teased.
Marvel also previously released a look at Hulk's gladitaor armor for Thor: Ragnarok (see below).
"Thor: Raganarok" has a November 3, 2017 release directed by Taika Waititi starring Chris Hemsworth as Thor, Mark Ruffalo as Hulk/Banner, Tom Hiddleston as Loki, Idris Elba as Heimdall, Cate Blanchett as Hela, Tessa Thompson as Valkyrie, Jeff Goldblum as Grandmaster, Anthony Hopkins as Odin and Sam Neill in an unknown role.