There could be a group of characters. Maybe a new team of benders who are on a quest with the Avatar. For example, a water bender with a special ability that is crucial to the story, a fire bender with a mysterious past that ties into the 'flame in the dark' aspect, and an earth bender who provides stability and wisdom to the group.
Well, perhaps it has a concept of a different kind of spiritual connection. In the official Avatar series, the spiritual aspect is very important. In this fanfiction, it could be that there are new ways of interacting with the spirits, like a more direct or unusual form of communication that changes how the Avatar's powers are perceived and used.
Well, the concept of 'The Dark One' in Avatar The Last Airbender fanfiction can vary a great deal. It could be a spirit that has been corrupted and now spreads darkness and chaos. This entity might be able to manipulate the elements in a perverted way, unlike the normal bending seen in the show. For example, it could create dark versions of the four elements that harm rather than help. Fanfiction writers often use such a character to explore darker themes and create more complex storylines for the beloved Avatar world.
Characters from the spirit world, such as Raava and Vaatu, might also be popular in Korra Dark Avatar fanfiction. Since the concept of the Dark Avatar is closely related to the balance between light and dark in the spirit world, these spirit entities can be used to drive the story forward, either as allies or foes of the Dark Avatar.
There could be a fanfiction where the Dark Avatar tries to find redemption. This story could show the internal struggle of the character as it battles its own dark nature while being influenced by the good around it. It would be a complex and emotional journey.
In some fanfictions, The Dark One can create illusions on a large scale. These illusions are not just simple visual tricks but can also affect a person's other senses. For example, it could make a group of people feel extreme cold when there is no real change in temperature, or it could make them hear voices that drive them insane. This is a very powerful ability as it can disorient and defeat opponents without direct physical confrontation. Also, it could be able to absorb the bending abilities of those it defeats, making it stronger with each victory.
In this fanfiction, the OC reborn as the dark avatar might find themselves in the middle of a power struggle. There could be other powerful entities vying for control or trying to eliminate the threat of the dark avatar. The OC has to navigate this complex web of politics and power while also dealing with their own internal conflict about their new form and the darkness within.
One interesting Dark Avatar Aang fanfiction could be where he uses his newfound dark powers to challenge the traditional order of the Four Nations. Maybe he decides that the old ways are holding the world back and tries to reshape it in a more radical way.