I think the BnM: You have a pretty good idea about what to do with a singular Hulk movie.
The idea I would add is really make the story physiological. Like, Banner is trapped and he himself believes his interplanetary banishment is justified. He gets trapped in the Hulk persona, because he wants himself to die...but then something gives him hope again.
In Planet Hulk, I liked the idea that his blood somehow allowed life to grow again. This is an interesting twist...and could be a catalyst for Banner to hope again, but now, the beast and man are struggling in an competition with each other...how to get out alive. It is like whose solution will work...and their minds are actually learning to trust each other and meld, but you see it as a struggle for survival.
It would be damn hard to write...and even harder to shoot...but if you pulled it off, it would be an amazing movie.