Often, the main character from the original work tends to be popular in fanfiction. They are the ones readers are already familiar with and interested in seeing more of. So, it's likely that the central figure in 'through a mirror darkly' would be a popular choice.
In many cases, characters with mysterious or complex personalities are popular in fanfiction. In 'through a mirror darkly fanfiction', perhaps a character who has a hidden agenda or a dual nature would be among the most popular. They offer a lot of room for exploration and interpretation by the fanfiction writers.
One common theme is the exploration of duality. Since it involves a mirror, there's often a contrast between the normal world and the mirror world, which can represent different aspects of a character or society. Another theme is the unknown and the fear that comes with it. The mirror world is often dark and mysterious.
In 'through a mirror darkly fanfiction', a great plot idea might be to introduce a new character who has the power to manipulate the mirror itself. This new character could be either a villain or an ally. If a villain, they could use the mirror to cause chaos and confusion among the existing characters. If an ally, they could help the others understand the true nature of the mirror and use it for good. This would add a whole new layer of intrigue to the story.
Daenerys Targaryen is also a favorite. Her journey as a powerful woman can be reimagined. Maybe in the mirror fanfiction, she takes a different path in her conquests, or her relationship with her dragons has a new aspect to it. There are so many possibilities for her character development.
I'm not certain without reading it, but I bet it has versions of the classic Star Trek characters like Kirk and Spock, just in their mirror universe incarnations. These mirror versions would likely have very different personalities compared to their normal counterparts.
One interesting 'through the mirror darkly fanfiction' story could be one that explores the alternate realities more deeply. For example, a story where the characters find a way to manipulate the mirror to their advantage in the darkly world. Another could be a story that focuses on the psychological impact of constantly seeing the distorted world through the mirror.
One main theme is often the exploration of alternate realities. Since 'a mirror darkly' has that element, fanfictions tend to expand on it, showing different versions of the story's world.
One popular character is Ash. He could enter the Mirror World and have his whole perception of Pokemon battles and relationships challenged. His Pikachu might also have some interesting interactions with the mirror - versions of other Pokemon.
I'm not entirely sure as the 'through a glass darkly story' isn't a widely known story with a common set of established characters that everyone is familiar with. It could be that there is a protagonist who is on a journey of some sort, perhaps a seeker of knowledge or truth.
I'm not sure specifically as I haven't read this particular novel in great detail. But usually, in a novel with such a title, there would be a protagonist who is on a journey of discovery. There might also be supporting characters like friends or family members who either help or hinder the main character's exploration.
Captain Archer in the mirror universe is often a popular character. His different moral stance compared to the normal Archer makes for interesting stories.