You can try Archive of Our Own (AO3). It has a large collection of fanfictions, and you can search specifically for Danny Phantom fanfictions where Dash finds out about something related to Danny. Just type in relevant keywords like 'Danny Phantom Dash finds out' in the search bar.
One idea could be that Dash gets in trouble and Danny Phantom, instead of using his powers to embarrass him like he usually might, decides to help him secretly. This could lead to an unlikely friendship between them as Dash starts to suspect that there's something different about Danny but can't quite put his finger on it.
It can develop in various ways. It might start as a hostile relationship, with Dash bullying Danny as usual. But as the story progresses, they could find themselves in a situation where they need to cooperate, like when facing a powerful ghost. This shared experience can gradually turn their relationship into one of mutual respect.
One great Danny Phantom Jazz Dash fanfiction is 'Jazz's Discovery'. In this story, Jazz starts to suspect Danny's secret life as Phantom and her journey of discovery is really engaging. It has a great mix of family dynamics and the supernatural elements that are central to the Danny Phantom universe.
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.
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.
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 possible storyline could be Danny facing a new, more powerful ghost while also dealing with his regular high - school drama. In this fanfiction, the Dash x Danny aspect might add a layer of unexpected friendship or rivalry as Dash discovers Danny's secret identity and has to decide whether to support or expose him.
A plot could be that Dash discovers Danny's secret identity as Danny Phantom. At first, he's shocked and a bit scared. But then, he realizes that Danny has been using his powers for good. He decides to keep the secret and even helps Danny out in some small ways, like covering for him when he has to disappear to fight ghosts. This new - found alliance between them could grow into a real friendship over time.
Power dynamics is also a frequent theme. Given Danny's ghost powers and Dash's position as a jock or bully, fanfictions often explore how these power differences play out. It could be about Danny using his powers to stand up to Dash in new and unexpected ways, or about Dash coming to respect Danny's powers. There are also some fanfictions that focus on the theme of redemption, where Dash realizes his wrongdoings and tries to make amends.