The Legend of Korra - Book 2: Spirits

 Spoiler

My only issue that I think about with a "Dark Avatar" is just the fact that Vaatu would be reincarnated. Period. He would not have all four elements to bend. Raava was the one who collected the other elements from the Lion Turtles and she is the one holding onto all four at once. As far as we know, Vaatu has not collected any from the Lion Turtles. That's my only drawback about the whole thing.