One idea could be that the Pokemon trainers gain Shinigami powers. For example, Ash could discover he has the ability to purify Hollows in the Pokemon world, which are like dark or corrupted Pokemon. His Pikachu could also have a unique transformation related to this new power, like having a Zanpakuto - like form.
Maybe the Soul Reapers could be sent to the Pokemon world to investigate strange disappearances of Pokemon souls. Ichigo and his friends would have to team up with Pokemon trainers to solve the mystery. They might be shocked by the variety of Pokemon and their unique abilities, and the Pokemon trainers would be in awe of the Soul Reapers' powers.
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 that Ichigo from 'Bleach' somehow gets transported to the world of 'Inuyasha'. He might meet Inuyasha and they could team up to fight a common enemy, like a powerful demon that has somehow gained the powers of Hollows from 'Bleach'. Their different fighting styles would make for an exciting combination.
One idea could be a scenario where Ichigo and Naruto meet in a strange dimension. They initially misunderstand each other but then realize they have a common goal of protecting their respective worlds. They could team up to fight a new threat that somehow has the power to destroy both the Soul Society and the Hidden Leaf Village.
One interesting idea could be the combination of powers. For example, the Soul Reaper's zanpakuto abilities mixed with the unique Semblances in RWBY. Maybe a character from Bleach using their Bankai in the world of RWBY and how that would interact with the Grimm. Another idea might be the character interactions. How would Ichigo get along with Ruby? Their personalities are quite different but they both have a strong sense of justice. It could lead to some really cool dialogue and development.
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.
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 a crossover with the Harry Potter universe. Pokémon could be integrated into Hogwarts, like having Pikachu as a familiar for a wizard. Another could be a crossover with the world of Final Fantasy. Pokémon trainers could team up with Final Fantasy characters to battle a common enemy. A third idea is a crossover with the Marvel universe, where Pokémon might have unique powers when exposed to the Infinity Stones.
A Bleach and One Piece crossover would also be cool. Imagine the Straw Hat Pirates arriving in the Soul Society. Luffy might try to befriend some of the Soul Reapers, and Zoro could have an epic sword - fighting match with one of the more skilled Soul Reapers. Usopp could also be amazed by the different spiritual powers at play.
A cool concept would be to combine the magic systems. So, the Soul Reapers' Kidou could be related to Fairy Tail magic in some way. For example, a Soul Reaper could teach some of the Fairy Tail mages how to use Kidou for a special mission against a common enemy that has the ability to disrupt both the Soul Society and the Fairy Tail world.
Another idea is to have the Death Eaters somehow get access to Hollow powers. This would create a whole new level of threat in the Harry Potter universe. The characters from Bleach could then be called in to help the Order of the Phoenix deal with this new threat. Harry Potter might learn some skills from Ichigo about combat and vice versa.