While the rest of Hollywood was going wild for superhero movies and shared universes in the 2010s, James Cameron was keeping his nose pressed firmly to the grindstone trying to figure out how the heck to make a sequel to his Oscar-winning, box office monster "Avatar." The easy answer is "You don't," but it's clear by now this sci-fi franchise is no mere cash cow for Cameron (well, okay, it's not just that) -- it's his true calling.Still, by the time the first of Cameron's four planned sequels, "Avatar: The Way of Water," arrives at the end of 2022, it will have been a whole 13 years since the