One idea could be to have the characters from Kingdom Hearts end up in the world of Nier Automata. Sora could be exploring the post - apocalyptic landscapes, and his keyblade might react in strange ways to the technology and magic present in Nier's world. He could team up with 2B and 9S to fight against the common threats like the machines. The Heartless could also be introduced into Nier's world, creating new challenges for both the heroes of Kingdom Hearts and Nier Automata.
First, you need to do your research on both universes. Know the lore, the characters, and the settings of Kingdom Hearts and Nier Automata. This will help you create a believable crossover. Then, think of a central plot. It could be something as simple as a shared enemy or a more complex idea like the merging of the two worlds' magic systems. For example, how would the Heartless interact with the machines in Nier Automata? Once you have the plot, start introducing the characters. Decide how they will meet and what their initial reactions will be.
The different settings also pose a problem. Kingdom Hearts has various Disney - inspired worlds and its own original worlds, while Bleach is mainly set in Soul Society, Hueco Mundo, and the human world. Merging these settings in a believable way requires a lot of creativity. For example, how would a character from Bleach react to a world like Agrabah from Kingdom Hearts? It's not easy to make it seem natural.
One popular 'Kingdom Hearts and Bleach crossover fanfic' is 'Soul Reapers in the Keyblade War'. It has an interesting plot where characters from Bleach get involved in the Keyblade War scenario of Kingdom Hearts. The author does a great job of blending the two universes' magic systems and character abilities.
One interesting concept could be a crossover with the Final Fantasy series. You could have characters from Kingdom Hearts interact with iconic Final Fantasy characters in a new world. For example, Sora and Cloud could team up to fight a new threat. Another concept could be a crossover with Disney's Aladdin. Picture Sora and Aladdin going on a new adventure together in the world of the Magic Carpet. A third concept might be a crossover with the Legend of Zelda. Link could join forces with Sora to save both their worlds from a common enemy.
One great 'Kingdom Hearts Crossover Fanfiction' is 'Kingdom Hearts and Final Fantasy: The Convergence'. It combines the elements of Kingdom Hearts and Final Fantasy in a really interesting way. The characters from both universes interact in a believable manner, and the plot keeps you hooked from start to finish.
First, decide which other franchise you want to cross over with. Then, think about how the characters from both universes will interact. For example, if it's a Kingdom Hearts and Star Wars crossover, consider how Sora would react to the Force. Next, come up with a plot. It could be a simple adventure to save a new world that combines elements of both universes. After that, start writing the introduction, introducing the main characters and the setting.
Maybe Sora, Donald, and Goofy end up in the Naruto universe. They would be really confused at first. Sora could try to understand the concept of chakra and how it's different from the magic he knows. They could team up with Naruto's team to take on Akatsuki members who have somehow been influenced by the darkness in the Kingdom Hearts world.
One possible plot could be that Sora and his friends find themselves in the Star Wars universe. They team up with Luke Skywalker to fight against the Heartless that have somehow invaded the galaxy. Sora's Keyblade could be a unique weapon in this new setting, and they could learn about the Force while sharing their own knowledge of the Heart.
First off, think about what kind of story you want to tell in the Kingdom Hearts universe. Maybe it's a new adventure for the main characters or a side story about a minor character. Make sure your writing style matches the tone of the original series and keep the magic and fantasy alive throughout.