According to
Geek.com, Hulk’s involvement ties into another rumor that’s been circulating for a long time – the possibility of seeing
Planet Hulk on screen:
“
Loki and Hela’s plan involves the major arc of the Marvel Universe, and just when it looks like Thor is about to put a stop to it, he gets banished — but not to any old planet. He ends up on a gladiatorial planet where his head is shaved and he’s forced to fight for his life. Let’s call this planet Sakaar, like it was in the comic books.
Thor knows that he has to get to some MacGuffin weapon object to defeat Hela and Loki (could this be the Infinity Gauntlet that Thanos is in possession of, or maybe an assemblage of Infinity Stones?) and he has to get off the planet. Thor runs through all of his alien opponents in gladiatorial combat, even without his trademark hammer. Finally, Thor has to face the current champion of the arena, and wouldn’t you know who that ends up being? The Hulk.”
The report goes on to say that the Hulk hasn’t reverted back to Bruce Banner since
Avengers: Age of Ultron, but the two personas are beginning to merge, and so the Green Goliath recognizes Thor. It’s from here that Hulk and Thor begin the cosmic road trip the film has been billed as.