One popular trope is the 'fish out of water' story. For example, when a Harry Potter character is suddenly in the One Piece world, they have to figure out how things work. Another is the power - exchange trope, like a One Piece character getting some form of magic ability.
A very popular trope is the 'portal' trope. A magical or mysterious portal opens up between the two universes, allowing characters to travel back and forth. Then there's the 'mentorship' trope. Maybe an older and wiser character from Harry Potter, like Dumbledore, could mentor a young One Piece character. Or a powerful One Piece pirate could teach a Harry Potter character about the ways of the sea and piracy.
The 'team - up' trope is quite common. This could be Harry and Luffy joining forces to face a common enemy. Also, the 'culture shock' trope, where characters from one universe are shocked by the customs and norms of the other. For instance, a One Piece pirate being amazed by the wizarding world's use of magic for everyday tasks. There are also fanfics that explore the idea of a character from one series learning the skills of the other, like a Harry Potter character learning swordsmanship from a One Piece character.
One popular trope is the 'fish out of water' scenario. This is when a character from Tokyo Ghoul ends up in the wizarding world of Harry Potter and has to adapt. For example, Kaneki suddenly finding himself in Hogwarts and being completely bewildered by the concept of magic. Another common trope is the power - up or power - combination trope. Where a character combines the powers of a ghoul with magic from Harry Potter to become super - powerful. For instance, using a spell to enhance the strength of a kagune.
The crossovers of characters are the best. Imagine Zoro dueling with a Death Eater or Hermione teaching Chopper some advanced magic. These interactions not only show the differences in their abilities but also how they can complement each other. It's also interesting to see how the world - building elements of both One Piece and Harry Potter are merged. For example, how the One Piece islands might be affected by the presence of magic or how the Hogwarts houses could be related to the different crews in One Piece.
In Harry Potter fanfiction, a common theme is 'what if' scenarios. For example, what if Voldemort had won? In Joker fanfiction, the theme of chaos and the breakdown of society is popular. One Piece fanfiction often has themes of friendship and the pursuit of dreams, similar to the original series.
In 20 years of Harry Potter fanfiction, there are several popular tropes. The 'Powerful!Harry' trope is quite common. Fans like to write about Harry having even more power than in the books. Also, the 'Soulmate' trope is popular, especially when it comes to romantic pairings. And the 'Dark!Hermione' trope has emerged, where Hermione takes a more morally ambiguous or even dark path, which is an interesting twist on her character.
The 'Power Transfer' trope is often seen. This is when a character from one series gains the power or ability from the other series. Say, Harry getting the ability to see ghosts like Sam does in Supernatural. And then there's the 'Shared Enemy' trope. A threat that is dangerous to both the wizarding world and the Supernatural world forces the characters to cooperate, creating exciting storylines.
A very common trope is the 'Dark!Harry' concept. In these stories, Harry turns to the dark side, either because of being mistreated or for some other reason. It's interesting to see how the story unfolds when the main hero becomes an anti - hero. Also, the 'Teacher - Student Romance' trope exists, though it's a bit controversial. But it does add an element of forbidden love to the stories.
One popular trope is the 'alternate universe' where the story takes place in a different setting or with different rules from the original series. For example, a wizarding world where Voldemort never existed. Another is the 'character redemption' trope, like Draco Malfoy getting a chance to be a good person from the start. And 'romantic pairings' that fans love to explore, such as Hermione with someone other than Ron.
One common trope is the 'fish out of water' situation where Castiel is suddenly in the Harry Potter world and has to figure out how things work. Another is the 'powerful ally' trope where he becomes a crucial help to Harry and his friends.
Harry Potter and Luffy are often popular to combine. Harry's leadership in the wizarding world and his determination to fight against evil can be paired well with Luffy's natural leadership and his never - give - up attitude as a pirate captain.
One Piece's Zoro is quite popular in these fanfictions. His sword - fighting skills can be compared and contrasted with the magic - based combat in Harry Potter. Ginny Weasley from Harry Potter is also a favorite. She can be written as having adventures in the One Piece world, perhaps even falling in love with a One Piece character. And of course, One Piece's Sanji with his unique fighting style and chivalrous nature can make for great stories when put in the context of the Harry Potter universe.