Well, perhaps the Omnitrix gets corrupted by Hollow energy. Ben has to seek help from the Soul Reapers in the Bleach world to fix it. In the process, he discovers that there are some similarities between the different forms in his Omnitrix and the Zanpakuto spirits. This leads to an exchange of knowledge and power - Ben learning some Soul Reaper techniques and the Soul Reapers getting insights into the alien technology of the Omnitrix.
A really cool plot could be that the Hollows in the Bleach universe start evolving at an abnormal rate due to some strange energy emissions from Ben's Omnitrix. Ichigo and Ben have to figure out a way to stop this evolution. Ben uses his different alien forms to combat the new, more powerful Hollows while Ichigo tries to find the source of the problem within the spiritual world. Along the way, they encounter various challenges and make new friends and enemies in both universes.
One possible plot could be that Ben, with his Omnitrix, accidentally lands in the Soul Society. He gets mistaken for an intruder at first. Then, he teams up with Ichigo to fight against a new common enemy that is threatening both the world of Bleach and the places Ben has to protect. His alien forms could provide unique abilities in battles against Hollows or other powerful foes in the Bleach universe.
One possible plot could be that Goku somehow gets transported to the Bleach universe. He might initially be confused by the concept of Hollows and Soul Reapers. But then, with his power, he could start fighting Hollows, thinking they are some sort of new villains. His ki-based attacks would be a new and powerful force in the Bleach world, and he could end up teaming up with Ichigo to fight a common, extremely powerful enemy, like a super - powerful Hollow that even the Soul Reapers are having trouble with.
Well, a cool plot might involve a Hollow in 'Bleach' escaping to the world where Lucy exists. Lucy, being the curious and strong - willed character she is, decides to track it down. Along the way, she meets some Soul Reapers who have also come to deal with the Hollow. They realize that Lucy has a special connection to the spiritual energy that could be used to defeat the Hollow more effectively. So, they join forces, with Lucy learning some Soul Reaper techniques and them learning about her world's magic.
The plot of such a fanfic might involve a villain who has the powers of both the Akatsuki (from Naruto) and the Arrancar (from Bleach) and is causing chaos in both worlds. Naruto, with his Rinnegan, and Ichigo are the only ones who can stop this villain. Naruto could use his Rinnegan - related abilities like the Six Paths powers to combat the villain's dark techniques. Meanwhile, Ichigo uses his Bankai and Hollow powers. The two of them would have to learn to work together, overcoming the differences in their powers and fighting styles.
Another interesting pairing could be Kevin from Ben 10 and Byakuya. Kevin has his unique powers related to absorbing and manipulating energy, which could contrast with Byakuya's refined and traditional Soul Reaper skills. They could start off as adversaries, perhaps misunderstanding each other's motives, but then come to respect and work together as they realize they are facing a common threat that requires both their skillsets to overcome.
Another plot could be that an evil force in the RWBY universe discovers the existence of the Omnitrix from another dimension. They manage to create a device to pull Ben and the Omnitrix into their world. Ben has to adapt quickly to this new world while Team RWBY, who are initially suspicious of him, realizes that he could be an ally against this new threat. They join forces, with Ben using his alien forms in creative ways to combat the Grimm and the evil force's minions.
One interesting plot could be that Ben, using his Omnitrix, accidentally lands in the Pokemon world. He teams up with Ash and Pikachu to stop an evil organization that is trying to use both Pokemon and alien technology. For example, they might be creating hybrid creatures using Pokemon DNA and alien powers. Ben's different alien forms could have unique interactions with the Pokemon. His Four Arms form could be really strong in Pokemon battles, while his XLR8 form could be used to quickly traverse the Pokemon world to reach different areas in time to stop the villains.
One idea could be that Ben discovers a strange Poké - Ball - like device during one of his alien adventures. When he activates it, it transports him to the Pokémon world. There, he teams up with Ash and Pikachu. His Omnitrix malfunctions due to the different energy in the Pokémon world, and he has to find a way to fix it while also helping Ash battle against Team Rocket, who are now interested in the power of the Omnitrix.
One possible plot could be that the Omnitrix somehow gets linked to the Green Lantern power source. Ben could end up with a new set of alien forms that are influenced by Green Lantern energy. For example, he might have an alien form that can create constructs like a Green Lantern but with its own unique alien twist.
Well, perhaps the Death Note from the Bleach universe gets mixed up with the magic in Harry Potter's world. A powerful wizard could get hold of it and start using it in ways that disrupt the balance of life and death in both worlds. Harry and his friends would have to figure out how to stop this misuse, with the help of some Bleach characters who understand the true nature of the Death Note.