Aang made for a heartwarming hero in Avatar: The Last Airbender and his successor, Korra, had an emotionally devastating journey leading the world into the modern era. Both heroes were completely different from one another, and every insight into past Avatars like Roku, Kyoshi, Kuruk and Yangchen showed just how diverse and unique the masters of all four elements could be.
However, as different as they all were, every Avatar in the franchise had one crucial thing in common: they were heroes. The Avatar franchise is ripe for a new Avatar that shows just how devastating that power can be when its wielder is no hero at all.
The moral complexity of Avatar is one of the defining traits of the franchise that makes it stand out among stories geared for children that still maintain 100% of its appeal to adults. The original series' first villain, Zuko, underwent one of the greatest redemption arcs in television history as the series proved centrally interested in what it is that motivates its villains and how they can change. Even the Avatars are not above moral relativism, as virtually every named Avatar suffered repercussions for their actions that, while well-intentioned, brought unintentional negative consequences to the world. So what happens when an Avatar actually does intend such harm?
Just because the stories in the Avatar world so frequently feature the full range of the moral spectrum doesn't mean the franchise is unfamiliar with outright evil villains. Ozai stands as the most prominent example, getting about as cackling cartoon villain as antagonists can come, but numerous other villains have sought the destruction of the world, selfishly amassed powers that threatened those around them and cared little for moral consequence. Exploring just how devastating the power of the Avatar could be when used by one of these villains would provide a challenge for the heroes like no protagonist prior had to face.
Challenging the heroes of the story would be one of the chief virtues of an evil Avatar tale. Any prolonged story in Avatar previously necessitated increasingly strained excuses for the hero not to immediately resolve the narrative by employing the Avatar State to their advantage. Whether they had yet to master the Avatar State or lost access to it, the more media that centers on an Avatar as its protagonist, the greater the creative difficulty in explaining why the most powerful person in the world is challenged at all. By making the Avatar the villain, the challenges the Avatar State presents end up servicing the narrative rather than hindering it.
As part of the lore, it would also go a long way in explaining why Avatars are not immediately hailed as great heroes fighting for the side of justice throughout their world. If the Avatar line of succession had nothing but a sterling reputation of heroes in its history, it's harder to believe that those who stand in opposition to the Avatar can really justify their positions. Even one evil Avatar would besmirch the reputation of their line of succession so that antagonism toward the Avatar becomes much more justified. And that's just if the story takes place as a prequel -- there's just as much, or perhaps even more, creative possibility in making one of the future Avatars after Korra an evil Avatar unlike any seen before.
A story in which benders of every element must unite against the figure who is supposed to defend the harmony between them writes itself. It even sidesteps the usual quest to master the elements that repeated for Aang, Korra and Kyoshi throughout the franchise. An evil Avatar would tell a completely new story while expanding on and enriching the stories that came before -- exactly what the Avatar franchise needs.