A time - travel fanfiction would be interesting. Danny and Star accidentally travel back in time to a historical period, say the Victorian era. They have to adapt to the different social norms and find a way back to their own time while also getting involved in a mystery or a love story within that era.
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.
A more complex plot could be that there is a prophecy about a union between a half - ghost and a magical being like Star. Danny and Star initially don't believe it, but as they keep crossing paths and helping each other out, they start to see that there might be something more to it. However, there are forces that oppose this union, thinking it will disrupt the balance between the human world, the magical world, and the Ghost Zone. So they have to fight against these forces while also coming to terms with their own feelings.
One common theme is friendship. Since they are two characters, often their relationship develops into a strong bond of friendship. They support each other through various difficulties.
The plot could change in terms of her love interests. Instead of the typical male - led romantic arcs, there could be new female - centered ones. For example, she might have a different type of relationship with a male character, where she takes the lead more often.
In this fanfiction, it could be that Danny Phantom meets a Danny Phantom from a parallel fan - made story. At first, they don't trust each other. But as they learn more about each other's worlds, they realize that there are some common elements that are being threatened. It could be that a powerful ghost is trying to rewrite all the fanfic universes related to Danny Phantom. So, they must combine their strengths and knowledge to stop this from happening.
A plot could involve Obi - Wan having a vision of a future where he and Siri are married and they bring about a new era of peace in the galaxy. So, he convinces Siri to marry him. However, the Sith sense this change in the Force related to their relationship and try to stop it. Obi - Wan and Siri then have to fight not only the Sith but also the misunderstandings within the Jedi Order. They may also meet some other Jedi who have had similar experiences with love and get support from them.
One idea could be that the OC is a new student in town. The OC has some unique powers that are a mystery even to themselves. Danny and Star notice the OC acting strangely and decide to investigate. Danny, being half - ghost, senses something different about the OC. Star, with her bubbly personality, tries to befriend the OC first. As they get closer, they discover that the OC's powers are linked to an ancient ghostly artifact that Danny has been researching. Together, they embark on a journey to figure out the origin of the OC's powers and protect the town from the potential threats that come with the artifact.
One common theme is the exploration of their different worlds. Danny comes from his own world with his own set of rules and powers, and Star has hers. Fanfictions often explore how they bridge these differences.
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.