One interesting plot could be Valerie finding out about Danny's ghost powers in a very public and dangerous situation. Maybe they are in the middle of a fight with a powerful ghost and Danny has no choice but to reveal himself to save Valerie. Another plot could be Valerie slowly suspecting Danny over time as she notices little things about his behavior that seem strange in relation to the ghost attacks. For example, he always seems to disappear right before a ghost shows up.
Well, obviously Danny and Valerie are the main ones. Danny, with his dual identity as a high school student and a ghost - fighting superhero. Valerie, the ghost hunter who is always on the lookout for any spectral activity. Tucker and Sam, Danny's best friends, often play important roles too. They might be the ones trying to protect Danny's secret from Valerie or they could accidentally be the cause of Valerie finding out.
In this fanfiction, Nathan could be a new student in town who has a strange connection to the Ghost Zone. Valerie initially suspects him of being a ghost or working with them. But as she investigates, she realizes that he is actually trying to close a dangerous portal to the Ghost Zone that was accidentally opened by one of his experiments. Valerie then decides to help him, and during the process, they face many challenges and dangerous ghosts. Through this adventure, they form a deep bond and become an important part of each other's lives in the Danny Phantom universe.
One interesting storyline could be Valerie discovers Danny's secret identity in a really unexpected way, like during a ghost attack where she's about to capture him but then realizes who he really is. It adds a whole new layer of drama to their relationship.
Danny is often very well - written. Writers can explore his dual identity further, like how he struggles to balance his normal life and his ghost - fighting duties. In fanfiction, we might see more of his internal thoughts when dealing with Valerie, especially when he's trying to keep his secret from her.
In a Danny Phantom fanfiction involving Valerie and Nathan, they could become allies in the fight against ghosts. Maybe Nathan discovers Valerie's secret identity and offers his technological expertise to help her upgrade her ghost - hunting gear. This would make them a formidable team, taking on powerful ghosts that Danny might have trouble with on his own.
In this kind of fanfiction, Maddie might initially be shocked. She could feel a mix of disbelief and horror that her son Danny is a half - ghost. But then, she might start to see Danny in a new light. Maybe she'll realize all the times he was trying to protect the family from ghosts while hiding his secret.
In a common 'Danny Phantom' fanfiction where Paulina finds out, the plot often begins with Paulina being in the right place at the right time. For instance, she could be near the Fenton portal when Danny is coming out in his ghost form. At first, she is terrified, thinking he is some kind of monster. But as Danny tries to explain his situation, she slowly starts to understand. This new knowledge then affects their relationship in school, with her having to keep his secret and also dealing with her own feelings about this new side of Danny. Sometimes, it can also lead to her getting involved in Danny's battles against other ghosts, either by accident or by choice.
One idea could be a story where Danny and Valerie have to team up to fight a new, powerful ghost that neither can defeat alone. They start off distrusting each other but as they face challenges, they learn to work together and gradually develop feelings for each other.
You can try Archive of Our Own (AO3). It has a large collection of fanfictions for various fandoms, including Danny Phantom. Just use the search function and type in 'Danny Phantom and Valerie Gray' to find relevant stories.