Typically, one of the main plot elements is the clash of different power systems. For instance, the Semblances in RWBY and Devil Fruits in One Piece. This often leads to interesting battles and strategic thinking.
One idea could be that the Straw Hats end up in the Naruto world and have to team up with the ninja to fight a common enemy. For example, they might face a powerful demon that can only be defeated with both Devil Fruit powers and ninja jutsu. Another idea is that some of the Naruto characters get Devil Fruits. Like Naruto getting a Logia type fruit that allows him to transform into a powerful energy form, similar to his Nine - Tails chakra mode but with new abilities.
Another great one is 'One Piece Meets RWBY: A Grand Adventure'. In this fanfic, the Devil Fruits from One Piece add a new layer of complexity to the battles in the RWBY world. It's really exciting to see how the characters from both series adapt to each other's presence and use their combined powers to overcome difficulties. For example, Luffy's rubber abilities combined with Ruby's speed make for some really cool fight scenes.
Luffy and Naruto would be an amazing pairing. They both have a strong sense of justice and are extremely determined. Luffy's carefree nature could balance out Naruto's sometimes - overzealous attitude. Another interesting pairing would be Zoro and Sasuke. They are both powerful swordsmen with a dark past and a drive to become stronger. They could have some epic sword battles and also learn from each other's fighting styles.
Well, it could be about the Straw Hats accidentally ending up in the Marvel universe. They meet the Avengers and at first there's some misunderstanding because of their different ways. But then they realize they can work together. Maybe Luffy is fascinated by Thor's hammer and tries to lift it, while Zoro wants to have a sword fight with Captain America's shield.
One common theme is the clash of combat styles. For example, when Zoro crosses over to a world like Naruto's, his swordsmanship clashes with ninjutsu. Another theme is adaptation. Zoro has to adapt to new environments, like if he's in a world with different magic systems or cultures.
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.
One characteristic is the mash - up of power systems. You get to see Bleach's reiatsu - based powers and One Piece's devil fruits and Haki combined. For instance, a character might have a zanpakuto and also be able to use Armament Haki. Another characteristic is the interaction of different character personalities. The serious and often brooding characters from Bleach meet the more carefree and adventurous ones from One Piece, creating unique dynamics.
One characteristic is that they often expand on the existing world of One Piece. For example, they might explore uncharted islands or hidden lore. Another is that they can focus on different relationships between characters. Fanfic novels may also introduce new devil fruits or unique abilities that are not in the original story.
First, you need to have a clear concept. Decide how you want to merge the two universes. For example, will Pokemon be native to the One Piece world or are they brought in from another dimension? Then, create engaging characters. Maybe a trainer who is also a pirate wannabe. Outline your plot, like a quest to find a rare Pokemon that also has some connection to a One Piece mystery.