One possible plot is that Blackbeard discovers a portal that leads to a world full of mythical creatures from different mythologies. He decides to capture some of these creatures and bring them back to the One Piece world to use them in his plans for domination. For instance, he might try to tame a dragon from European mythology or a giant beast from Asian mythology. Another plot could be that Blackbeard's crew mutinies against him and he ends up stranded on an island in a crossover world, like the world of Final Fantasy. There, he has to learn new skills and magic to get back to his own world and take revenge on his crew.
One interesting plot could be Blackbeard traveling to an alternate universe where the roles in the One Piece world are reversed. For example, pirates are the law - enforcers and the Marines are the outlaws. Another plot could involve Blackbeard teaming up with a character from a different anime in a crossover event. Maybe he partners with Naruto to search for a powerful treasure that has somehow ended up in the Naruto universe. A third plot could be Blackbeard losing his powers and having to start from scratch in a world where all the other One Piece characters have forgotten who he is, and he has to regain his status and power in a new and unexpected way.
Blackbeard would probably be very interested in the magic in the Fairy Tail world. He might try to steal some of the powerful magic items or learn magic for himself. He could see the mages as a threat to his plans if they oppose him, but also as potential allies if he can manipulate them. He would likely use his cunning and power - hungry nature to his advantage in this new world.
Another interesting plot might be Sakura getting involved in a battle against the Marines. She could use her strength and combat skills, which are quite different from the One Piece characters, to create unique fight scenes. Maybe she teams up with Zoro and they exchange sword - fighting knowledge in some way. Also, her chakra abilities could be an interesting addition to the devil fruit powers in the One Piece world.
Sherlock Holmes could be an exciting crossover. Imagine the logical and observant Holmes trying to solve a mystery that involves Blackbeard's pirate activities. Blackbeard, on the other hand, would be trying to outwit Holmes and continue with his piratical ways. It would be a clash of two very different worlds - the world of piracy and the world of Victorian - era detective work.
One interesting concept is a crossover with other popular anime like Naruto. Imagine Luffy and Naruto teaming up, it would be a clash of two very different power systems and personalities. Another concept could be a crossover with a superhero universe. For example, Luffy meeting Superman. Their powers are so different, it would make for an exciting story.
One idea could be that the Straw Hats end up in the Naruto world somehow. Maybe they land on the Hidden Leaf Village's outskirts. Luffy could befriend Naruto immediately because of their similar natures. Naruto would be intrigued by Luffy's Devil Fruit powers and Luffy would be amazed by Naruto's jutsu. Another idea is a battle tournament where characters from both universes participate. Zoro could face off against Sasuke, which would be an epic sword - against - sword and Sharingan - against - three - sword - style fight.
Perhaps Natsu could be from a previously unknown Devil Fruit - eating tribe in the One Piece universe. His journey could involve him searching for a way to return to his own world while getting involved in the various adventures and politics of the One Piece world. He might even team up with Luffy to take on a powerful common enemy.
Another concept is that the Devil Fruits in One Piece could be related to the chakra system in Naruto. For example, a character from Naruto accidentally eats a Devil Fruit and has to adapt to this new power. Or perhaps the One Piece itself is a powerful object that can also be used to control chakra in some way. The story could follow the characters as they explore this connection and deal with the chaos it causes in both worlds.
A cool 'One Piece Brook Crossover Fanfiction' idea is a crossover with the 'Lord of the Rings' universe. Brook could journey to Middle - earth. His undead nature might make him an object of both fear and fascination. He could use his music to soothe or terrify the various creatures he encounters, like the hobbits might be initially scared but then grow to like his music. Also, a crossover with 'Naruto' could be interesting. Brook could interact with the ninja, and his unique abilities would surely stand out among the shinobi.
One idea could be that Luffy and Toriko have a cooking competition using the strange ingredients from both their worlds. Another could be a joint adventure where they search for a mythical ingredient that is said to grant eternal strength and is hidden on an island in the Grand Line. Or perhaps a situation where the villains from both universes team up, forcing Luffy and Toriko to combine their forces to defeat them.
One idea could be that Ichigo and Luffy team up to fight a common enemy that has the powers of both the Hollows from Bleach and the Devil Fruits from One Piece. For example, a powerful pirate who has somehow harnessed Hollow powers and is causing chaos in the Grand Line.