Having roamed through the jungles of Central and South America, I can tell you that invading that much territory, and holding it, would be impossible. Why the hell would we invade South America anyway? Aside from taking out that idiot Chavez, there isn't much reason for it. Besides, we do actually have some friends in South America. A war to take all of that would leave an army of exhausted soldiers and worn out gear.
As far as regional threats becoming big powers, China would be my number 1. I wouldn't put it past Russia either. Yeah, they are hurting right now, but there are some there that are calling for a return to Communism.