Perhaps a spell goes wrong in the Harry Potter universe and merges the two worlds partially. So we see Hogwarts students having to interact with the Addams family members in a shared space. Some students would be terrified while others, like the Weasley twins, would be fascinated and want to learn from the Addams family's unique brand of magic and eccentricity.
One idea could be that the Addams family somehow ends up in the wizarding world of Harry Potter. Maybe Wednesday Addams could be a transfer student to Hogwarts. She would be really unique among the students, with her strange interests and dark sense of humor. She might not be too impressed with the magic at Hogwarts as she has her own 'creepy' ways of doing things at home.
One idea could be that the Addams family visits Hogwarts. Wednesday could be intrigued by the Dark Arts classes and start causing some spooky chaos among the students. She might even try to out - do Draco Malfoy in being menacing.
A fanfiction could be set in a world where Hogwarts has an exchange program with Nevermore Academy (Wednesday's school). Harry Potter is chosen to go to Nevermore, and Wednesday is sent to Hogwarts. At Hogwarts, Wednesday has to deal with the magical chaos and the rather more optimistic students compared to her own schoolmates. Meanwhile, Harry has to navigate the strange and often macabre world of Nevermore, and they both learn a lot from each other's worlds.
One idea could be a story where the Addams family moves to Forks. Wednesday Addams could befriend Bella, and they end up getting into all sorts of strange adventures together. Maybe Wednesday helps Bella deal with the vampires and werewolves in her own unique, macabre way.
One idea could be that Hulk somehow gets transported into the wizarding world of Harry Potter. Maybe a magical experiment gone wrong by a Hogwarts student creates a portal that sucks Hulk in. He could then cause chaos as he's not used to the magic and strange creatures there.
One idea could be a story where a Wendigo somehow enters the wizarding world of Harry Potter. Maybe it's brought in by a dark wizard who wants to use it as a weapon. The Trio - Harry, Ron, and Hermione - would then have to figure out how to defeat it using their magic and knowledge.
A possible concept is that of a Hogwarts student who discovers they are related to the Addams Family. This student could bring some of the Addams' values and aesthetics to Hogwarts. For example, they might decorate their dormitory in a more gothic and spooky way, much to the bewilderment of their roommates. They could also introduce some of the Addams' unique forms of 'entertainment' like their strange and macabre family games. And it would be interesting to see how the teachers at Hogwarts react to this new addition with their Addams - like quirks, especially when it comes to classes like Defense Against the Dark Arts, where the student's view of 'dark' might be a bit different from the norm.
Another possibility is that Voldemort creates a spell that combines the dark elements of his magic with the curse from Beauty and the Beast. The Trio then has to find a way to reverse it. They might discover that the solution lies in the same kind of selfless love that breaks the Beauty and the Beast curse. This could lead to some really interesting character development and plot twists.
One idea could be that the Space Captain crash - lands on the magical world of Harry Potter. He has to learn about magic to repair his ship while Harry and his friends are intrigued by the advanced technology the captain brings. Maybe they team up to fight a new threat that is endangering both the wizarding world and the space captain's galaxy.
A more complex fanfic could involve a dark wizard from the Harry Potter world trying to use the power of the music and the happy atmosphere in Mamma Mia as a way to hide from the Aurors. The characters from Mamma Mia would initially be unaware of the danger but gradually team up with the Harry Potter crew to defeat the dark wizard. This would combine the magic of Harry Potter with the sunny, carefree setting of Mamma Mia.
One idea could be a crossover where Harry Potter characters end up in the world of Prince of Tennis. For example, Harry could befriend some of the tennis players and learn about their passion for the sport. Maybe he could even try his hand at tennis, using his magic to add some unique twists to the game.