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.
The Addams family could fit into the wizarding world by bringing their own sense of style and values. Their mansion could be a place where students go for a different kind of magical experience. They might hold parties where they showcase their strange magical artifacts. In terms of the Hogwarts houses, they might not fit the typical mold. Wednesday could be a unique Slytherin who doesn't care about the pure - blood - mania but rather about her own personal power. Pugsley could be a Hufflepuff who is loyal to his family's odd ways. And the family as a whole could contribute to the diversity of the wizarding world.
Morticia would likely be intrigued by Harry. She might be drawn to his mysterious past in the wizarding world and his magical abilities. Gomez, on the other hand, would probably be excited to have a new family member to share his adventures with. Wednesday might see Harry as an interesting addition, someone who could bring a different kind of darkness to the family. And Pugsley might look up to Harry, especially if Harry shows him some cool magic tricks.
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.
Imagine Pugsley Addams getting a Hogwarts letter. He'd be so excited to learn magic, especially dark magic. He could get into trouble by trying to create his own spells inspired by the Addams family's unique interests. Meanwhile, the Addams family's butler, Lurch, could be a half - giant like Hagrid. He could interact with Hagrid and they could share stories about taking care of strange creatures.
If Harry Potter were raised by the Addams family in fanfiction, his style of dressing would be completely different. Instead of the traditional Hogwarts robes, he might be seen in something more gothic, like the Addams family's style. Also, his sense of humor would be darker. He'd probably find the pranks at Hogwarts a bit too tame compared to what the Addams family gets up to.
It would be really interesting. Harry would grow up in a very different atmosphere. Instead of the strict but loving environment at the Dursleys, he'd be in the spooky and unique Addams family setting. He might learn some very unusual magic from them, and his personality could be a blend of the Addams' eccentricity and his own Gryffindor courage.
Well, the Addams Family might be intrigued by Harry's magic wand. Morticia could compare it to one of Gomez's strange collectibles. Wednesday might see Harry as an interesting new addition to her rather dark world view. And the Dursleys? They would be terrified if the Addams Family ever showed up to check on Harry. It could be a really wild and wacky crossover with lots of unexpected interactions between the two very different fictional universes.
In some fanfictions, there could be a plot where the dark magic in the Harry Potter universe intersects with the Addams family's natural inclination towards the macabre. Maybe a dark wizard tries to use the Addams family's home as a base for some evil plan. But the Addams family, with their own brand of strength and eccentricity, turn the tables on the wizard. This could lead to an alliance between the Addams family and the Hogwarts faculty to fight off the common threat.
You can start by looking on major fanfiction platforms like Archive of Our Own (AO3). It has a vast collection of fanfictions from various fandoms, and there's a good chance you might find something related to this crossover there. Another option is FanFiction.net, which has been around for a long time and also hosts a large number of fan - created stories.