I forget where, but someone mentioned killing off an Avenger in Avengers 2. I suggested that Ultron funds a way to kill off Banner, which creates a Hulk that can't calm down as there is no physical Banner to revert to. After Hulk Smashes Ultron, he turns on the Avengers forcing them to launch him into space and his own movie. Banner lives on in his head and eventually grasp how to be a conscious Banner Hulk as that was the only way to coexist since he couldn't revert back into human Banner.
Post Planet Hulk, Hulk somehow hitched a ride back to Earth by the Guardian's in GotG2 (post credits?) and arrives back on earth just in time for Avengers 3 with Banner now fully in control of the Hulk.
I haven't read all the comics, so I don't know if there are better stories for The Hulk, but I think my idea seems easy enough to do if they were hoping to do Planet Hulk and have a Banner Hulk be the result in the end.
It's been done before, where Banner took a bullet to the brain and couldn't calm down (because, hey! Bullet in the brain) and they couldn't surgically remove the bullet because they couldn't get through the Hulk's skin. The Hulk and Banner have been split twice now and there was even one where Banner committed suicide in his own mind so there was only Hulk left. But those stories weren't even interesting in comic form, so I don't think they'd really translate that well to the big screen.
And believe, me, with that "suicide in his own mind" one, I cite it as evidence that having the mindless Hulk for a significant length of time is fucking boring. It was about three or four years straight of just mindless, bestial Hulk, no talking and even though it ended up with the re-emergence of Banner as it turned out that mind suicide doesn't actually work, it didn't make it any better.
Besides, there are easier ways to get a merged Hulk that don't also having an entire movie that primarily features a mindless raging Hulk. In the comics, just forcing Banner to face his demons while under hypnosis did the trick, but I imagine that mind control powers or magic could accomplish the same thing. I can easily see Loki merging the personalities as a way to neutralize the Hulk's innate fighting skills (or force Banner to the monster all the time), for instance, or Doctor Strange merging the personalities to help fight Thanos in a more strategic manner.
As far as best Hulk stories, I've given this a lot of thought, and I really think that the best way to handle a film Hulk story is to treat Banner like a supervillain. He can't just be sitting around minding his own business until the bad guy shows up because he does not react well to surprises. He needs to be the instigator, needs to be the one with the plan. My favorite Hulk stories have always been the ones where Banner was trying to get something done at the same time as dodging the authorities.
The Norton Hulk almost did this, but they made the mistake of having him half-assed try to get something unattainable, which made the movie kinda fail. I'd say that given Banner is very much anti-establishment, having him discover something seriously shady that's the government's been doing and go quasi-domestic terrorist would be a more interesting take (this has also been done before in comics, but I suggest it because it worked very well). They probably will never do this, but I can dream.
