[Minor Spoilers for Thor: Ragnarok Ahead.]
Thor: Ragnarok is the start of the separation process for Hulk and Bruce Banner, according to Mark Ruffalo. While the aforementioned Marvel characters are played by a single actor, Marvel Studios has continued to explore their dynamic with one other, highlighting their complicated relationship. Although this will continue to be the case moving forward, the two will be slowly pulled apart - with Banner retaining his human mindset while the Hulk develops his own consciousness.
Ruffalo's Hulk and Banner may never get a solo film (at least not in the foreseeable future), but the Marvel Cinematic Universe has thus far, been able to use Ruffalo's on-screen character effectively in ensemble films like the Avengers movies. As he joins his first ever team-up project with Chris Hemsworth's Thor, we get to spend more time with the green raging monster and the scientist that houses him in Ragnarok.
Sitting down with Collider while doing the press rounds for Ragnarok, Ruffalo revealed that Hulk and Banner will continued to grow apart from each other from hereon out. He also hinted at where their complicated relationship will go in the MCU next:
"This is where it starts, and it starts- it's basically a mini version of 'Planet Hulk.'" So the fans get that, and then we start to build out the separation between the Hulk and [Bruce] Banner. Now the Hulk can be his own character, and Banner can now start to be the person that he would be without having to be afraid of being excited all the time. So we start to get to know these two characters in a way that we didn't get to do before. A part of that is having a speaking Hulk as, like a 6-year-old boy."
The separation is evident, following the Hulk's reign over Banner's physical body for the two years since Avengers: Age of Ultron. Banner is not at all thrilled when he finally wakes up in Ragnarok, finding himself on an alien planet after having been (essentially) unconscious this whole time. If anything, Ragnarok approaches the Jekyll and Hyde angle in different ways - playing it for laughs at times, while also highlighting Bruce's fear of turning into the green rage monster again and never able to come back to his human state.
Thor: Ragnarok, in the process of doing that, sets up the next two Avengers - Avengers: Infinity War and the still-untitled Avengers 4. After that, it's not clear where either Hulk and/or Banner will end up (which is also the case for most of the MCU's principal characters). That said, it seems like Ruffalo knows where he wants his character trajectory to go, having already had talks with top Marvel execs about what lies ahead for Banner and his always smashing being alter-ego.