It depends on how they handle the new plot. If they introduce interesting new concepts and develop the existing ones further, it will be exciting. But if they deviate too much or make it too predictable, it might not be as good as the original.
One popular theme is exploring the backstories of the characters in more depth. For example, delving into Yuu's life before he transferred to Hoshinoumi Academy. Another theme could be alternate universes where the events in the anime take a different turn, like if the ability users were not persecuted. Also, there are fanfictions that focus on the relationships between characters, such as the budding romance between Yuu and Nao, expanding on their interactions and feelings in ways not fully shown in the anime.
Yuu is definitely one of the most frequently written characters. His complex backstory and powerful ability make him an interesting subject. Nao is also popular as she is a strong - willed female character with her own set of abilities. Another character often written about is Joujirou, with his unique power and his role as a friend and ally in the story. Writers like to explore their relationships and develop their personalities further in fanfiction.
In Charlotte Anime After Story, we could see more of the unique abilities that were introduced in the original series. There might be new plot twists related to these abilities. Also, it could explore the backstories of some secondary characters more deeply.
One of the main features is the detailed world-building. It creates a vivid setting that adds depth to the story. Also, the relationship dynamics between the characters are quite interesting and drive the narrative forward.
Charlotte is not a true story. It's a work of fiction that takes viewers into a fantasy world with unique settings and characters. The story was crafted to captivate and engage the audience's imagination rather than being based on actual events.