While journeying across the Nine Realms with the God of Thunder in Thor: Ragnarok, Ruffalo discussed with Collider the arc Bruce Banner is dealing with regarding his relationship with The Hulk and how that’s going to resolve itself. He explained:
No one’s really gone in that place. I mean, he’s always been running from him or trying to deny it, but they’re coming for a collision course with each other, and I think that can be pretty exciting if we can figure out a way to do it multi-dimensionally.
In The Incredible Hulk, Banner was desperate to find a cure for his “condition.” By The Avengers, he seemed to have made peace with his dual nature and simply did his best to contain it. In Avengers: Age of Ultron, he continued to use Hulk as a weapon for good, but after he was traumatized by Scarlet Witch‘s mental control, he later fled from Black Widow and his allies during the climactic battle against Ultron to keep them safe. It sounds like when we reunite with him in Thor: Ragnarok, the two sides will finally battle with each other, something that’s been years in the making, both in the real world and within the MCU.