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 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.
In a fanfiction, Wednesday Addams could visit Hogwarts. She would probably find the castle fascinating, but also a bit too cheery in some parts. She might make friends with some of the darker - themed students, like those from Slytherin. And she could introduce them to her pet spiders and other creepy - crawlies. Harry would be intrigued by her and her family's attitude towards death and the afterlife, which is different from what he has learned in the wizarding world.
A possible crossover could be in the area of magic. The Addams family might have their own brand of 'weird' magic that clashes or combines with the magic in Harry Potter. For instance, Pugsley Addams could create some really odd potions using ingredients from both the Addams' world and Hogwarts' supply.
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.
First, you need to do your research on both universes. Know the characters well, their backgrounds, their personalities. For Harry Potter, understand the magic system, the Hogwarts houses, etc. For Wednesday Addams, know her family, her love for the macabre. Then, think of a creative way to bring them together. It could be a portal between their worlds or some sort of magical accident. When it comes to the 'lemon' part, build up the relationship gradually. Don't rush into it.
There are quite a few. A great one might involve Morticia Addams' gardening skills being used to grow some very strange and magical plants in the Hogwarts greenhouse. The Addams family values of being different and unafraid of the dark side could be a great contrast to the more 'normal' wizarding families in the Harry Potter series. This can create a lot of interesting storylines, like how the Addams kids deal with the rules and traditions at Hogwarts.
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.
Well, Wednesday would likely be very matter - of - fact in her interactions. She doesn't have time for small talk. Harry, used to the more chatty and friendly environment of Hogwarts, would have to adjust. But as they interact more, Harry would see that Wednesday has a sharp mind and a strong will. Wednesday would see Harry's courage and determination. For example, if they were in a dangerous situation in the wizarding world, like facing a group of dark wizards, Wednesday's ability to stay calm and think rationally in the face of danger would be an asset. And Harry's magical skills would be useful to Wednesday, who has no wizarding training. They'd form a unique bond based on mutual respect for their different strengths.
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.