The worlds can be blended through character crossovers. For example, Vaan from Final Fantasy XIII could visit Hogwarts and be amazed by the magic there. He could try to learn magic spells, while also teaching the wizards about his world's combat skills.
Another way is to blend the magic systems. In Harry Potter, magic is cast with wands and spells, while in Final Fantasy XIII, there are summons and crystal - based powers. A fanfiction could create a scenario where a character discovers a way to use wand magic to enhance the power of the crystals or vice versa. This could lead to new and exciting plotlines as characters explore the combined power sources. Also, the different races in Final Fantasy XIII like the Pulse and Cocoon races could interact with the magical races in Harry Potter such as wizards, witches, and goblins. They could form alliances or have conflicts based on their different cultures and abilities.
The cultures of the two worlds could be explored in the fanfiction. The hierarchical society in RWBY with the Huntsmen and Huntresses could interact with the monarchy and different social classes in Final Fantasy 15. This could lead to political intrigue and new storylines as characters from both worlds try to understand and adapt to each other's cultures.
Well, in Final Fantasy 8, magic can be drawn from enemies and junctioned to stats. In a Harry Potter - Final Fantasy 8 fanfiction, this could mean that wizards start to learn how to draw magic from magical creatures in the Forbidden Forest in a similar way. They could then use this 'drawn' magic to enhance their own abilities, like Hermione using the magic she draws from a hippogriff to become faster at casting spells.
The worlds can be blended by having Barbie visit Diagon Alley. She could be amazed by all the magical items and use her fashion sense to customize her wizarding robes. For instance, adding some sparkly elements that are typical of Barbie's style.
The blend can occur through the use of magic in relation to trigonometry in solving problems. For example, in a fanfiction, a magical creature is lost in a forest that is magically generated with trigonometric patterns. The characters need to use their knowledge of both magic and trigonometry to find the creature. Also, a character's magical power could be related to their understanding of trigonometry, so as they learn more math, their magic becomes stronger and more versatile.
One way to blend Naruto and Final Fantasy 15 in fanfiction could be through character crossovers. For example, Noctis could meet Naruto. Noctis has his unique combat skills with his weapons and magic, while Naruto has his ninja techniques. They could learn from each other. Maybe Noctis could teach Naruto some magic tricks, and Naruto could show Noctis how to be more stealthy like a ninja. This would create interesting interactions and new storylines.
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 great way to blend the worlds is through a shared threat. A villain that uses both magic and technology to cause chaos. This would force Harry and Tony to work together. They would have to learn from each other's strengths and overcome their differences. Harry could teach Tony about the subtleties of magic, while Tony could show Harry how to use technology to gain an edge in battle.
In fanfiction, the characters could also meet during a special event. For instance, there could be a magical music festival that attracts both wizards and Muggles. Harry Potter and his friends are there for the magical elements, while Hannah Montana is performing. They cross paths and end up collaborating on a song that has magical properties, like making everyone who hears it feel extremely happy.
One way is through a portal. For example, a magical portal could open up between Hogwarts and Neverland. This allows characters from both worlds to interact. Harry Potter could enter Neverland and be shocked by the lack of traditional magic but also find unique magical elements there related to the island's magic. Captain Hook could enter Hogwarts and be amazed and also threatened by the powerful magic there.
Themes can be merged. The theme of good vs evil is present in both. In fanfiction, the dark wizards from Harry Potter could team up with the more malicious vampire clans in Rosario Vampire to create a super - evil force. And the heroes from both worlds would have to join forces to stop them. This would require characters to look past their initial differences and prejudices to fight for the greater good.