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.
Another concept could be that the Soul Reapers in Bleach are sent to investigate the abnormal spiritual activity in the world of My Little Nightmare. As they explore, they discover that the nightmares are actually manifestations of corrupted souls, and they must find a way to purify them using their Soul Reaper abilities.
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.
One idea could be a crossover where Naruto and Hinata somehow end up in the Bleach universe. Maybe they are mistaken for powerful spirits and are recruited by the Soul Society. They could use their ninja skills in battles against Hollows. For example, Naruto's Rasengan could be a powerful attack against Hollows, and Hinata's Byakugan could help in detecting spiritual presences.
One idea could be a crossover where characters from Naruto, Shiba (assuming you mean the Shiba clan from Bleach) and Bleach meet in a new dimension. For example, Naruto characters could be transported to the Bleach universe and interact with the Shiba clan members. They could team up to fight a common enemy, like a new powerful Hollows or Akatsuki members who have also found their way into this new world.
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.
One idea could be that the Omnitrix somehow gets mixed up with the magic of the sirens. Ben, as one of his alien forms, has to deal with the alluring powers of the sirens in the My Little Pony world. Maybe he has to stop them from using their voices to take over Equestria while also not getting charmed by them himself.
A more complex idea could be that there's an alternate universe where the villains from both worlds combine forces. The Green Goblin teams up with Discord. In this fanfiction, Spiderman and the Mane 6 have to put aside their differences in fighting styles and cultures to defeat this powerful alliance. They learn from each other's strengths, with the ponies teaching Spiderman about the magic of friendship and Spiderman showing them some cool combat moves.
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 that Marinette (from Miraculous Ladybug) discovers a magical seashell that transports her to the underwater world of The Little Mermaid. There, she meets Ariel and they team up to fight an underwater villain who is stealing the magic of both the surface and the seabed. Maybe the villain is after the Miraculous and Ariel's voice for some dark purpose.
One idea could be that the Phantom, with his mastery over music, somehow finds himself in the underwater world of The Little Mermaid. He might be intrigued by the mermaid's unique voices and decides to create a grand underwater opera with them. The mermaid protagonist could be both fascinated and a bit scared of this strange, masked figure from the surface world.
One idea could be Naruto is transported to the world of Little Witch Academia. He could attend the academy, but with his ninja skills, he'd be a very different kind of student. Maybe he'd try to use chakra instead of magic at first and create some interesting chaos.