Well, perhaps in the fanfiction, the succubus uses her powers to bring out hidden desires among the members of the harem. For Danny, this could mean facing some internal struggles as he tries to maintain his heroic image while also being part of this strange new dynamic. The plot could revolve around how he reconciles these two aspects of his life.
One idea could be that Danny Phantom, while dealing with his normal ghost - fighting duties, attracts a succubus who then creates a harem of various characters from his world. Maybe they all have to work together to fight off a greater evil that is attracted to the combined energy of the harem.
Another plot idea is that the OC is from a different dimension and is brought to the Danny Phantom universe by accident. Their strange and alluring appearance and behavior draw in different characters. The OC might initially struggle to fit in but gradually forms relationships with them, which could turn into a harem situation as they each have their own reasons for being attracted to the OC, like the OC's knowledge of different technologies or magic from their home dimension.
Maybe Danny gets transported to a different dimension where there is a harem - like society. The local girls there are intrigued by his ghost powers and his heroic nature. They start following him around, and he has to figure out how to deal with their attention while also trying to find a way back home. Some of the girls might be from different species, which could add more interesting elements to the story, like different cultural norms regarding relationships.
One idea could be Danny Fenton having to deal with a new ghostly enemy that can manipulate time, making his double life as Danny Phantom even more chaotic. Another could be Danny trying to balance his high school life, including exams and relationships, while also facing a ghost invasion that only he as Danny Phantom can stop. Or perhaps a story where Danny Phantom discovers a hidden power within himself that has a connection to his human side, Danny Fenton.
One idea could be that Danny discovers a new ghostly power that he uses to take revenge on a group of bullies who have been tormenting his friends. He could use his intangibility in a creative way, like sneaking into their lockers and making their things disappear.
The development of the characters can be really interesting. Danny might struggle with the new relationships and the presence of the succubus. He could be shown to have different facets of his personality come out, like a more protective side. The succubus could start off as a somewhat one - dimensional seductress but then be shown to have a complex backstory and motives. The other members of the harem could also have their own arcs. For instance, one might be trying to find their place in the group, while another could be dealing with past traumas that are healed through the new relationships in the harem.
One idea could be that Danny gets an illness that affects his ghost powers. For example, he might start losing control over his intangibility or invisibility. This could lead to some really comical or dangerous situations. Another idea is that his friends try to find a cure for his strange ghost - related illness while also keeping it a secret from the town.
Well, perhaps in the fanfiction, Undergrowth could create a parallel dimension filled with his overgrown plants and trap Danny there. Danny then has to navigate this strange new world, make allies among the plant - like creatures that are also being oppressed by Undergrowth, and find a way to break out and stop Undergrowth from spreading to the real world. This would be an exciting adventure full of mystery and discovery for Danny.
Maybe in the fanfiction, Danny's mom accidentally gets transported to the Ghost Zone and Danny has to go on a wild adventure to rescue her. Along the way, he discovers new things about his powers and his family's connection to the Ghost Zone. This could lead to some really emotional moments between Danny and his mom as they reunite and face new challenges together.
Well, a great plot for Danny Phantom Planet fanfiction could be about Danny's life after saving the planet. He might face new challenges like dealing with the fame that comes with being a hero. Some people could idolize him while others might be jealous or even fear him. You could also write about Danny trying to balance his normal life with his superhero duties on a more global scale. He could have to travel around the world to solve problems related to the changes that occurred during the Planet event.
Well, here's a plot. In a strange turn of events, Danny's ghost powers start to interact with alicorn magic in unexpected ways. This causes chaos in both worlds. The alicorn and Danny must work together to figure out how to stop the strange energy fluctuations. They might have to travel to different dimensions within their universes to find a solution. Along the way, they meet other characters from both franchises who either help or hinder their progress.