Corrupting one of the elements to fight for the Darkness would greatly spice this story up. That element must willingly give themselves to the Darkness out of a slight sway in their morals, not any of that magical or evil corruption bollocks, no. This betrayal could be influenced by an identity crisis, threatening to kill an element's friend or family, or worse: misguided love. Sort of like... what Pitch Black tried to do to Jack Frost in Rise of the Guardians, but instead of rejecting Damien's offer, they accept it. That would be exceptionally amazing since, well, Damien can be quite convincing.