Another interesting concept could be that Izuku's 'prince of darkness' status is a hereditary thing that has been hidden from him all his life. When it starts to manifest, he has to deal with not only the physical changes but also the political intrigue within the world of dark powers, where different factions either want to use him or destroy him.
A plot could involve the Prince of Darkness in a battle against other supernatural forces. He has to team up with unexpected allies, like a group of young witches who are also trying to protect their world from a greater evil. This would lead to a lot of character development as they learn to work together.
One idea could be Percy discovers he has a hidden connection to the powers of darkness that dates back to an ancient prophecy. Maybe he has to learn to control this new aspect of his powers while still being true to his friends at Camp Half - Blood.
One plot idea could be Issei facing a powerful new enemy that challenges his status as the Prince of Darkness. He might have to gather allies from different realms and use his unique powers in new ways.
A third plot idea is that there is a prophecy about the 'Prince of Darkness' in the Danny Phantom universe, and Danny is unknowingly fulfilling it. As he continues on his adventures, he starts to notice strange coincidences that seem to be leading him towards a dark destiny. He then has to decide whether to fight against the prophecy or try to understand it and use it for good.
Perhaps in this fanfiction, Izuku as a girl might face different challenges in the hero world. Society could have different expectations of a female hero, which would add a new layer to her character development. She might have to deal with sexism while trying to prove herself as a capable hero, just like real - world female heroes often do.
Some of the teachers at U.A. might also be antagonists. They could be secretly working for an organization that wants to control Izuku's powers for their own evil purposes. They would pretend to be helpful while actually setting traps and spreading misinformation about him.
One idea could be exploring the psychological changes of the characters during those 30 days. For example, how they cope with the lack of light and the isolation it might bring. Another could be creating new relationships or alliances that form specifically because of the 30 - day situation.
One idea could be exploring the backstory of the main character in 'Darkness of Birth'. Maybe they had a difficult childhood that led to their current state. Another idea is to create a different ending for the story, where the characters face different fates. For example, the protagonist could overcome their inner darkness in a more unexpected way.
You could focus on the relationship between the sparkle and a character who is initially afraid of it. For example, a young girl who sees the sparkle in the forest at night. She thinks it's something evil but as she gets closer, she realizes it has a warm and positive energy. Then, she devotes herself to spreading the light of the sparkle throughout her gloomy village.
Perhaps a fanfiction where the tiger of darkness has a human form as well. In this form, it tries to blend into society but still has its primal instincts. The story could be about the internal struggle it faces between its human side and its dark, tiger nature. It could fall in love with a human, and then the conflict would be about whether to reveal its true self or keep it hidden.