The Multiverse is the Idea that Millions, if not an Infinite number of Universes exist at the same time. Moving at different paces, occupying different dimensions and various other Plains of existance. Black is white, or Red, it could be a Bipedal walking three headed Elephant in some. Anywho . . . .
What if Aang went dark in the series. Roll with me for a bit, Justice Lord scenario. Say the Fire Nation , Azula, Combustion Man, whoever. Some Fire Nation loyalist gets a lucky shot in. Sokka pays the price for the Team, the Ultimate Price. Everyone Is distraught over the loss, They all band together in their Darkest of moments. They agree that The Fire Nation has taken far too much, that they need to be put in their place. . . .The grave. Even Zuko is on board. Together they refine their skills and even develop some dark techniques. So much left to cover. Bottom line Team Avatar ends The Fire Nation's Rule of terror and put the world under their rule, an Iron Clad Dictatorship.