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 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.
The OC could be a detective - like character who is investigating strange disappearances in Amity Park. Danny and Star get involved because they suspect ghosts are behind it. The OC doesn't believe in ghosts at first, but as they start to work together, they realize that there are things beyond their understanding. Danny has to be careful not to reveal his identity as Phantom too soon. As they dig deeper, they find out that there is a powerful ghost who is kidnapping people to use their energy for a dark ritual. The three of them must find a way to stop this ghost and rescue the captives.
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.
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 creating an OC who is a time - traveling ghost hunter. They could come from a future where the ghost situation in Amity Park has gotten out of control. This OC could have unique gadgets and skills specifically designed to deal with the more powerful ghosts that have emerged over time.
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, a great plot for Danny Phantom future fanfiction could be that Danny discovers a way to travel to different dimensions in the Ghost Zone. In these dimensions, he finds versions of himself with different powers and personalities. He then has to figure out how to use this knowledge to save his own dimension from a powerful ghost that is slowly draining the life force of the living world. It could involve a lot of self - discovery for Danny as he learns from his other - selves.
Maybe Ophelia has some sort of special power herself that she doesn't know about until she meets Danny Phantom. They could go on a journey to figure out what her power is and how it can be used to help the town. It would be a great way to explore both of their characters in a new light.
One idea could be that Danny discovers a new power that has been dormant all this time. Maybe it's a power related to time manipulation, which could lead to some really cool adventures where he has to fix time - related problems in the Ghost Zone or in the human world. Another plot could involve Danny meeting his future self who has come back to warn him about a great danger that is coming. This could create a lot of mystery and suspense as they try to figure out what the danger is and how to stop it.