The main villain could be a powerful underwater sorcerer who has been banished from the wizarding world long ago. He has been biding his time in the depths of the ocean, building his power. Now, he wants revenge on the wizarding world and also plans to take over Aquaman's kingdom. He uses a combination of dark magic and his knowledge of the ocean's secrets to create all kinds of disasters. Aquaman and Harry Potter must put aside their differences and work together to stop this powerful and vengeful villain.
Maybe a hybrid creature that is part sea monster and part dark wizard. This creature was created by an accident when a dark wizard was experimenting with underwater magic. It has the intelligence of a wizard and the strength and ferocity of a sea monster. It wants to rule both the magical world and the ocean world and sees Aquaman and Harry Potter as the biggest threats to its plans.
It could be a wizard who has gone mad with power and wants to control both the wizarding world and the underwater realm. He discovers a way to use dark magic to manipulate sea creatures and cause destruction in Aquaman's kingdom. He then sets his sights on Hogwarts as well, seeing it as another source of power.
One possible plot could be that Aquaman discovers a magical portal in the depths of the ocean that leads to the wizarding world of Harry Potter. He then teams up with Harry to fight a new threat that is endangering both the underwater kingdom and Hogwarts. Maybe it's a dark magic - using sea creature that has gone berserk.
One possible plot could be that Aquaman somehow finds himself in the wizarding world of Harry Potter. Maybe a magical spell gone wrong transports him there. He could use his unique underwater abilities to help the Hogwarts students in a task related to the Black Lake, like retrieving a powerful magical artifact that has sunk to the bottom.
Well, in a Harry Potter and Aquaman crossover fanfiction, we might see Aquaman's Atlantean magic interacting with Hogwarts' magic. For instance, the Sorting Hat might have a hard time sorting an Atlantean student brought by Aquaman. Also, Harry and his friends could go on an adventure to Atlantis with Aquaman as their guide, exploring the underwater world full of strange creatures and ancient magic.
Aquaman's ability to control the sea could be used to create massive water - based defenses around Hogwarts. In return, Harry could use magic to give Aquaman the ability to breathe air indefinitely when he's on land for extended periods. Their powers could also combine in a more offensive way. For instance, Harry could use a levitation spell on Aquaman while he's in the water, allowing Aquaman to attack enemies from above the water surface with his enhanced speed and strength.
It could be a Death Eater who has somehow made a pact with a powerful force from Scorpian's world. This villain would be extremely dangerous as they'd combine the dark magic of the Harry Potter universe with the unknown and potentially deadly powers from Scorpian's realm.
Lex Luthor from Supergirl's world could be the villain. He could somehow find a way to travel to the wizarding world and team up with some dark wizards. He wants to use magic to enhance his power and take over both worlds. Supergirl and Harry Potter would have to stop his evil plans.
Obviously, Kylo Ren would be one of the main characters. On the Harry Potter side, Harry Potter himself could be a main character. Their different views on power and good vs. evil could create a lot of interesting interactions. They could be at odds at first but then gradually come to understand each other.
It could be Aquaman. He is so different from the wizards in the Harry Potter universe. His fish - like abilities and his role as a king would make for some great interactions with the Hogwarts students and professors. He could teach them about the ocean and its secrets, while also being amazed by the magic of wands and potions.
Some smaller, more niche fanfiction communities and forums might also have 'Harry Potter Crossover Slash Fanfiction'. These can be a bit harder to find, but they often have a more dedicated group of fans who might create unique and less mainstream stories. You can try searching for 'Harry Potter fanfiction forums' and then looking for the relevant sections within those sites.
One common theme is the clash of different power systems. You have the magic from Harry Potter and the superpowers from the Avengers. Another theme is teamwork. It's about how the characters from both universes learn to work together. For example, Hermione might use her intelligence to help Iron Man with some tech - magic hybrid problem.