According to Geek.com, they have figured out how Hulk factors in and what his role is in Thor: Ragnarok will be, warning potential Spoilers ahead:
According to the site, Hela manages to destroy Mjolnir and banishes Thor from Asgard (again), but this time Thor Doesn’t end up on earth.. But on a gladiatorial planet instead. Determined to stop her and Loki, Thor must fight his way to escape this world, but when he runs into the current champion, he’s in for a surprise… It’s The Hulk.
“he’s managed to avoid reverting back to the Banner state since we last saw him in the Quinjet in Avengers: Age of Ultron. Hulk in Hulk form is pretty much unkillable, so he survived the trip to Sakaar (however he got there). His ‘Banner’ is starting to seep through into the Hulk persona, and because of the Hulk-Banner melding, Hulk actually remembers Thor and his relationship to him. That’s what starts the two on a Midnight Run to the MacGuffin that Thor desperately needs.”
So even though we couldn’t get a Planet Hulk film, it sounds like a version o the story is coming through in Thor: Ragnarok.