One idea could be that the characters from Bleach find themselves in the nightmarish world of Little Nightmares. They might have to face the grotesque creatures there using their unique powers from Bleach. For example, Ichigo could use his Zangetsu to fight off the long - armed janitor.
Maybe the powers in Little Nightmares are a result of some experiment gone wrong in the Bleach universe. So the main character of Little Nightmares is actually a victim of a Quincy experiment that backfired. This could lead to a story where the Bleach characters try to reverse the effects and save the Little Nightmares world.
Another interesting concept is that the Hollows in Bleach are somehow connected to the nightmares in Little Nightmares. The Soul Reapers have to enter the Little Nightmares realm to stop the Hollows from spreading their darkness further. They could team up with the Little Nightmares protagonist to achieve this.
Perhaps in this fanfiction, the Espada from Bleach are somehow related to the nightmares in My Little Nightmare. The Espada could be using the nightmares as a way to gain more power, and the protagonists from both worlds have to team up to stop them. This would create a really interesting dynamic between the two very different universes.
To write a great Bleach x My Little Nightmare fanfiction, focus on the contrast between the two worlds. Bleach has a more traditional shonen anime feel with its battles and power-ups, while My Little Nightmare is full of psychological horror and a sense of helplessness. Use this contrast to create tension and drama in your story. Also, introduce new elements that are a blend of both. For instance, what if a Soul Reaper's zanpakuto is affected by the nightmare world and starts to behave in strange ways? This can lead to some really exciting plot developments.
Ichigo from Bleach could be a main character. His strong sense of justice and powerful combat abilities would make him a great fit for the nightmarish world of Little Nightmares. He could be on a quest to save the innocent souls trapped there.
One idea could be a crossover between a horror movie like 'Nightmare on Elm Street' and a popular fantasy series such as 'Harry Potter'. Imagine Freddy Krueger showing up in Hogwarts, causing chaos among the students. Another could be a crossover with 'The Walking Dead', where the nightmares of the apocalypse blend with the already terrifying world of the undead. Or perhaps a crossover with a superhero universe, like 'Marvel', where the heroes have to face their worst nightmares in a new and unexpected way.
A great 'bleach crossover fanfiction' idea is to cross it with One Piece. The Straw Hat crew could end up in the Soul Society. Luffy's carefree attitude would contrast with the more solemn Shinigami. Also, a Bleach and Fullmetal Alchemist crossover where the alchemic principles are applied in the Bleach universe would be really interesting.
One idea could be a crossover where Naruto characters gain Bleach powers. For example, Naruto could have Ichigo's Zangetsu and learn to use it in his own ninja way. Another idea is to create a new character who is a mix of Naruto and Bleach elements, like having Sharingan - like abilities combined with a Quincy's powers. Or you could write a fanfiction where the world of Naruto is invaded by Hollows and the ninja have to learn Soul Reaper techniques to fight them.
Another interesting concept could be a character swap. Imagine Ichigo from Bleach attending the school in Highschool DxD. How would he interact with the devils and other supernatural beings there? He might use his Soul Reaper powers in unexpected ways within the Highschool DxD universe.
One idea could be to have Goku from DBZ end up in the Bleach universe and learn about soul reapers. He might mistake them for a new type of fighter and try to spar with them. Another could be a fusion of the power systems, like Ichigo getting the ability to go Super Saiyan on top of his existing powers.
One idea could be that Sasuke somehow ends up in the Bleach universe and teams up with Ichigo. They could face off against a common enemy that has powers related to both the Sharingan and the Zanpakuto. For example, a villain who can manipulate souls like in Bleach but also has the ability to copy techniques like the Sharingan.
Well, you could create an Arrancar OC that was once a human with a strong sense of justice. After becoming an Arrancar, they struggle with their newfound powers and the need to follow Aizen's orders. They might secretly help the Soul Reapers in some ways. Or perhaps an Arrancar OC with a power that is based on sound. They can use high - pitched frequencies to shatter objects or low - pitched ones to paralyze opponents. Also, an Arrancar who is an outcast among their own kind because they have a different view on the war between Soul Reapers and Arrancars could make for an interesting fanfiction.