The rumor about the Hulk appearing in "Thor: Ragnarok" has finally been confirmed but it only leads to another question: Why?
There are some sensible theories, as Cinema Blend stated, that the Hulk could be seeking a cure for his "condition." In the last scene of "Avengers: Age Of Ultron," we see the Hulk still at odds on whether he can still lead a normal life.
Seeing Hawkeye's secret life has pushed him to desire domestic bliss by settling down with Natasha (Black Widow). However, he is not sure if he can control himself, and that uncertainty can lead to dire consequences not just for him, but for everyone within a half-mile radius.
"If Bruce somehow ends up on Asgard, maybe he'll seek assistance from Thor's people in curing him of the Hulk," CB stated.
However, there are some holes to that theory. Thor is returning to Asgard in great haste because he sensed a catastrophe approaching in his dream/trance with Scarlet Witch. Why would he bring a mortal to Asgard if there would be grave danger, especially when they "cure" Bruce Banner-then he would no longer be the Hulk and unable to defend himself?
Thus, another theory may be more on point. What Culture mentions that Thor could have recruited Hulk to help him defend Asgard against the impending doom possibly caused by Thanos.
"It would make sense that Thor would get the biggest weapon available to him (who isn't involved in Civil War, since Vision would probably have come in handy too) to try and deal with Thanos himself when he comes after the Tesseract."
Thor knows about the Infinity Stones power and how they can override Asgardian science or magic-like when the Vision lifted his hammer. He understands that Asgardian power may be inadequate.
Which theory would hit home on November 3, 2017? We have almost two years to find out.