A more complex way to merge the worlds is to have a common enemy. For example, a new and extremely powerful dark force that threatens both the ninja villages in Naruto and the realms in Castlevania. This would force the characters from both worlds to work together. They would have to share their knowledge and powers. In this scenario, Naruto could teach the Castlevania characters about chakra, while they could teach him about dark magic defenses. It would be a story full of action and discovery as the characters adapt to each other's worlds and powers.
The worlds could be merged effectively by creating a common threat. For instance, a powerful entity that can only be defeated by the combined powers of both universes. This would force the characters from Naruto and Final Fantasy XIII to interact and work together.
A possible way for their worlds to merge in fanfiction is through a reincarnation plot. What if some of the Naruto characters were reincarnated in the Highschool DxD world? Naruto himself could be reborn as a devil or an angel. This would completely change the power balance and relationships in the Highschool DxD world. Rias would have to deal with this new Naruto, who still has his ninja spirit but now has to adapt to the rules and powers of the new world.
The worlds can be merged through a crossover device. For instance, a magical portal could open up between the two universes. This would allow characters from both to interact. The Bleach characters could bring their spiritual powers to the Criminal Minds world, and the agents could bring their psychological profiling skills to the Bleach world.
The cultures of the two worlds could be blended. For instance, the Pokemon Centers in the Pokemon world could be introduced to the Fire Emblem setting, serving as places for healing both soldiers and Pokemon. This would create a new aspect of world - building in the fanfiction.
A more creative approach could be a magical or time - travel based fanfic. Maybe a magic spell gone wrong sends Lady and Tramp into the world of the 101 Dalmatians or vice versa. This would force them to adapt to a completely new environment and interact with characters they wouldn't normally meet. It could also lead to some humorous misunderstandings as they try to figure out what's going on and how to get back to their own world.
Well, for starters, the settings can be combined. The high school from High School of the Dead can be the new location for Naruto characters. You can have Naruto - style classrooms where they learn not just regular subjects but also ninja techniques. And the zombies can be added to this mix as a new threat that the ninja - students have to face.
Well, you could give Jeff some ninja - like abilities. For example, his knife - wielding skills could be enhanced with chakra, making his attacks more powerful and deadly. And for Naruto, he could be influenced by Jeff's more chaotic and dark nature, which could make him question his own beliefs about good and evil. Maybe Naruto starts to have a darker side that he has to fight against, similar to how he battles the Nine - Tails' influence, but this time it's Jeff's influence.
The magic in the Harry Potter world could be linked to the religious - based powers of Alexander Anderson. For example, some of the spells in Harry Potter could be seen as having roots in the same kind of divine or spiritual energy that Anderson wields. This could lead to a situation where Anderson is either trying to understand the magic in the wizarding world as a different form of his own power, or he could be trying to convert wizards to his way of thinking about the power source.
You could also have a character who has a connection to both universes. Maybe a half - Naruto - half - Angeloid being. This character would act as a bridge between the two worlds. They would introduce the concepts of one universe to the other. For example, they could teach the ninja in Naruto's world about the advanced machinery of the Angeloid universe and at the same time show the Angeloid the importance of things like honor and loyalty in the ninja world. As this character moves between the two universes, the two start to blend together more and more.
One way to merge the storylines is through the concept of power. In Naruto, there is chakra, and in The Ancient Magus Bride, there is magic. They could find a way to exchange or combine these powers. Maybe a character from Naruto gets cursed and the only way to break it is by using the magic from The Ancient Magus Bride. This would force the characters to interact and learn about each other's worlds, thus merging the storylines. Also, the different cultures in both worlds, like the ninja clans in Naruto and the magical families in The Ancient Magus Bride, could be compared and contrasted, creating a rich and merged storyline.