No Hulk movie has really been a hit at the box office or with fans, and now the latest in regards to a standalone movie featuring the Bruce Banner and his alternate persona comes from Mark Ruffalo who feels it's his responsibility as an actor to portray a message.
As Mark Ruffalo only had about ten or twelve minutes of screen time in The Avengers, he wasn't able to do that, but speaking at the Sundance Film Festival, Ruffalo mentions if there was a standalone Hulk movie that it could offer to teach a lesson.
Ruffalo doesn't specifically state the Hulk movie would be "eco-friendly" (which brings up comparisons to Avatar), but says they have to take science and make it relatable.
Via Total Film:
As artists we have an ability to reach a lot of people in a deep way, and we have a responsibility. What we have to do as storytellers is to take science and make it relatable.
I think we have to tell the story of the positive and what we're going to try to do. Give them alternatives but don't make it a polemic.
I think that's the way to go, and it's a trend that'll start to develop. As a culture, when we latch onto a fad, we're actually working something out in the creative subconscious. The fact that all these doomsday and end-of-times movies are coming out is because we're actually working that out in the culture.
I'm in The Avengers, but if you took my screentime it would come to about ten minutes, along with the 12 other people I'm sharing the screen with, so my input's kind of limited on that. But if we get to a standalone Hulk movie, I'll have a much better chance to do that. I'm just learning now how to find my voice and bring it into the studio world.
Again, Ruffalo didn't say to turn the Hulk movie into some sort of liberal green party movie, but as Banner is a genius and scientist, it's possible they could somehow incorporate current science and technologies into the movie coupled with today's real world issues.
While I think making the Hulk literally go green would be a huge bomb at the box office ("Hulk save trees"), the Hulk is born out of radiation and Banner himself is an abuse victim (father abused mother etc), and it could be argued that Banner is, at least in the comics, a loner and a victim of bullying all of which are large themes being played out in the real world today.
It's also possible the Hulk could do missions for S.H.I.E.L.D. in a standalone Hulk movie that involves him going up against a terrorist organization that tries to damage and pollute the atmosphere (nukes, dirty nukes, typhoons, weather control etc.) without the movie directly coming out and implying global warming and controversial subjects of that nature. More an underlying message would probably be the best way to go.
Another possibility is to have the movie set during Planet Hulk, which has been rumored, where the Hulk saves (or does he?) the people of that planet from a tyrant. The only issue with going this route is that Banner wasn't involved. Of course that could be changed for the movie.