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.
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.
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.
Well, one thing that could go wrong is a misrepresentation of the characters' personalities. For example, if Ichigo was made to be overly timid when in the original he is brave. Another issue might be a jumbled plot. Maybe the fanfic tries to combine too many different storylines from different series in a way that makes no sense. Also, the power scaling could be off. If a minor character from another series is suddenly made much stronger than the main Bleach characters without proper explanation.
Definitely Ichigo and Ben. Ichigo has his strong sense of justice and his powerful Hollow - Soul Reaper abilities. Ben, on the other hand, has the Omnitrix with all its different alien forms. They would make an interesting duo as they could learn from each other's fighting styles and powers. Ben could learn about the spiritual aspect of Ichigo's powers, and Ichigo could be amazed by the physical transformations Ben can do.
Yes, there might be. There are often fanfics that combine different popular series. 'Busou Shoujo Machiavellism' and 'Bleach' both have their own unique elements, and some creative fans may be interested in creating a crossover story that blends the characters, settings or concepts from both. However, finding such a specific crossover fanfic might take some searching through various fanfic platforms.
First, know the source materials well. Really understand the characters, their personalities, and the rules of the Bleach universe. Second, plan your plot carefully. Don't just randomly combine elements from different series. Third, keep the tone consistent. If Bleach has a certain level of seriousness, don't make the crossover too comical in a way that doesn't fit.
One idea could be to have Ben Tennyson end up in the Young Justice universe by accident. Maybe his Omnitrix malfunctions during a fight and transports him there. He could then team up with the Young Justice heroes to fight a common enemy like the Light or some new villain that's a threat to both universes.
In a Ben 10 and Naruto fanfic, their powers would be a great combination. Ben has the ability to transform into various alien forms, each with unique powers. Naruto has his chakra and his powerful jutsu. They could learn from each other. For example, Ben could teach Naruto about some of the alien technologies, while Naruto could teach Ben about chakra control and the ninja way.