If we are to go by the IMDb listing for Thor: Ragnarok, WWE Superstar John Cena has joined the cast which is in part adapting the Marvel Comics Planet Hulk storyline.
Update: John Cena's name has since been removed from IMDb.
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So who is John Cena playing? Why it's himself.
I can actually see John Cena making a cameo as himself in the gladiator arena. While we don't know specifics about Thor: Ragnarok, I'm guessing Jeff Goldblum's Grandmaster kidnaps various "champions" from around the universe to do battle and compete in the arena (nod to the original Contest of Champions Marvel comic book event). It's possible that since John Cena is a "great" champion, that he gets picked for the gladiator arena and is featured in a scene. This also explains how Hulk gets taken off of Earth, as the Hulk would be considered a great champion, and how Thor makes his way to the arena.
John Cena playing himself in movies isn't unheard of as Cena was featured in the Fred movies and Daddy's Home as himself. John Cena also happened to voice the Hulk in the Marvel Avengers Academy video game.
"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.