I know what you're thinking: "This is just going to be another stupid thread about how we can bring back old antagonists who are clearly dead." But bear with me. Raava said that the defeated spirit would, eventually, be reborn in the other one and then emerge as a sperate being. So my theory is that over hundreds of Avatars, Vaatu would begin to blossom in Raava, and the Avatar would get darker and darker until there is a neutral Avatar. The next Avatar would then be two Avatars, identical twins. One would be a Light Avatar, and one would be a Dark Avatar. Potentially, each one would also get half of the elements. I.E., the Liavatar would get fire and water while the Daavatar would get earth and air. After they die, the cycle continues, with a new Daavatar and Liavatar born seperately. Potentially, of course, the Daavatar and Liavatar would each get all four elements.Â