Per the website JoBlo:
“Hot on the heels of NYCC, we’ve gotten word from our inside sources that THOR: RAGNAROK will see Chris Hemsworth’s Thor team up with none other than Mark Ruffalo’s HULK for the Odinson’s third solo film, which finds the God of Thunder facing what amounts to the Norse Apocalypse. Only this time out he’ll have some help from a fellow Avenger.
Mark Ruffalo joins Tom Hiddleston (also confirmed by our sources) and Jaimie Alexander in RAGNAROK, which we’ve also heard will involve a “distant planet” that is “not Asgard and not Earth.” What does that mean? Certainly the theory will be made that this is where Marvel will work in the long-rumored Planet Hulk storyline, and although we can’t completely verify that, we can’t completely deny it either. I’ll leave it to you on how you want to place your bets, but I know where my money is going. Regardless, Ruffalo is on board. As to how much time he’ll spend as Hulk vs. Banner, we can’t say, only that he’s absolutely part of the supporting cast and will feature prominently.”
Sounds like it could definitely be a way to incorporate the Planet Hulk storyline and have Mark Ruffalo in human form and have Thor to help round it out.