Here's a prospect: we get another actor, allowing Rambo to die. From that, we could get a sequel in the form of a prequel, following John Rambo on how he became what he was in "First Blood". That sequel, rather than having a comic book action movie tone would have one more in line with those serious 70s dramas, as I said, resembling possibly films like "The Deer Hunter" or "Platoon".
You have his life before the military, his military career, and his return home to work with, and you can make a somewhere along the line of his life before "First Blood". Such would also allow for a proper exploration of the plight of the Vietnam war veteran. Rambo was not a good role model, so to speak, by the time you got to the sequels. He was essentially the Hulk rather than an actual man, and the films were not concerned with the plight of veterans so much as punching Communists in the face. They also had the tendency to evoke and reinforce BS myths like the crazy veteran, the stab-in-the-back myth, the getting spit on and being called baby killer by Americans myth, etc, all of which were prevalent during the 80s and still today to all too large a degree. Very Right wing myths which play it as if Vietnam vets were against the world and the anti-war movement was wicked, where in reality Vietnam vets were largely the ones against the war and the VVAW was one of the largest protest organizations, the anti-war movement was largely sympathetic to GIs and the spitting thing is a myth in as much as it was not a common thing or even occasional thing, and the war was lost simply on it's own merits. The Rambo series reinforced all those myths.
One of the worst things it propped up, though, was the POW/MIA myth, where there was the notion that the Vietnamese still had American POWs in the (then) present, and that the United States government was ignoring it and covering it up. That myth got to be huge during the 1980s, when it reached the point where a majority of Americans believed it, and Rambo II played right into it and reinforced it and spread it further.
What a more serious Rambo series would have the chance to do is actually do something for the veterans of the Vietnam war and America in the aftermath of the Vietnam war. It could explore those difficult times, and shine a light on actual realities and struggles of America and of the Vietnam war and of the soldiers who went to the Vietnam war in the form of the Rambo character and his story. Instead, it turned Rambo into an action star, spread and reinforced myths of the Reagan era, all the while paying lip service to meaning something. I really have no problem with the Rambo series, though I know it may sound like that. But it's an action series which became one of those live action comic books in the 80s, and not what it could have been. And it did have all those BS Vietnam myths. With a critical eye, those features have to be brought up.
People like Kris Kristofferson, John Travolta, and even Dustin Hoffman were up for the part but objected to the violence either because of past war experience or on principle. Al Pacino was willing, but even his interpretation would have been very different.
