One idea could be a crossover where Danny Phantom and the Robin Brothers team up to fight a common enemy from their respective universes. Maybe a powerful ghost is causing chaos in Gotham and the combined skills of Danny's ghost powers and the Robins' combat and detective abilities are needed to stop it.
A more complex fanfiction could involve a time - travel plot. The Robin Brothers somehow get sent to Danny's world in the past, before he became Danny Phantom. They then have to help young Danny deal with some early signs of the ghostly problems that will later turn him into a half - ghost, while also trying to find a way back to their own time and world.
Another fanfiction could focus on a friendship that develops between Danny and one of the Robin Brothers. Danny could be visiting Gotham for some reason and accidentally gets involved in a case the Robins are working on. They initially have some misunderstandings due to their different backgrounds and ways of doing things, but over time, they become good friends and learn from each other.
A great plot twist could be that one of the Robin Brothers turns out to have a secret connection to the ghost world. Maybe he has a long - lost relative who was a powerful ghost and this connection is revealed during a battle against a major ghost villain. This would completely change the dynamics of the team - up and add a new layer of mystery.
A popular one is 'The Alliance of the Two Heroes'. In this fanfiction, Danny and Robin team up to fight a common enemy that is a threat to both their worlds. It delves into their different personalities and how they complement each other during battles.
Fanfiction.net also has a good number of these types of fanfictions. You might need to dig a bit deeper through the search results, but it's worth it. Just make sure to read the reviews and summaries to find the ones that suit your taste.
One idea could be that Danny and Sam have kids in the future and their kids inherit Danny's ghost powers. The story could focus on how they learn to control their powers while also dealing with normal kid stuff like school and making friends. It would be interesting to see how Sam, as a mom, guides them through this.
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.
One idea could be that Danny and Jinx accidentally cross paths in a dimension where the laws of magic and the supernatural are all jumbled. Danny is trying to find his way back home and Jinx, initially seeing him as a threat, later realizes they can help each other. They team up to navigate through this strange dimension and along the way, they learn about each other's powers and develop an unlikely friendship.
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.
Maybe they discover they are brothers through some supernatural or scientific means. At first, they don't get along as they have very different personalities. Danny is more laid - back while Damian is intense. But as they face challenges together, they learn to trust and rely on each other.
One idea could be that they discover their brotherhood through some magical event that links their two very different worlds. Maybe a magical object from the Harry Potter universe shows up in Amity Park and Danny's investigation leads to the revelation that they are related.
One idea could be exploring Danny's relationships in a different way. For example, how his friendship with Sam and Tucker might develop if they faced a new supernatural threat. Another could be a story where Danny discovers a new power and has to learn to control it while dealing with school and his ghost - fighting duties. And perhaps a fanfiction where Danny travels to an alternate dimension within the Danny Phantom universe.