Rambo 5 will see Stallone do battle against the Mexican cartels. But that wasn't always the case. The initial idea for the movie was to see the Vietnam Special Forces Vet do battle against a vicious monster in what would have been called Rambo V: The Savage Hunt. When that idea was scrapped they moved on to the current script where Rambo once again comes out of hiding to do mortal combat with a new set of militaristic humans, which honestly seems more in line with the recurring themes throughout the series.
The idea for his next movie is based on a story which Stallone owns the rights to that's called Hunter by James Byron Huggins. When it was initially planned as a Rambo sequel back in 2009, he explained that the story would have been about his character going up against "a feral beast". This would have been out of sorts for a First Blood movie and definitely seems like it should be its own thing. This is expected to be his follow up to Rambo 5.
They're currently looking for someone to pen the script.