It could be amazing. Picture Iron Man in the Soul Society, his high - tech gadgets contrasting with the spiritual energy there. Maybe Captain America's sense of justice would mesh well with the Soul Reapers' duty to protect. And Thor's godly powers might create some interesting interactions with the Hollows or other Bleach entities.
First, understand the power scales of both universes. You don't want to make one side too overpowered. Then, focus on character interactions. What would Wolverine say to Byakuya? Next, create a unique plot. Maybe a common enemy from another dimension is threatening both universes. This would force the characters to work together.
It's really hard to say who would be the most powerful. Saitama from One Punch Man (if we were to include him in this wild crossover thought experiment) would probably be a top contender just because he can defeat any opponent with one punch. But in a more traditional Bleach - Marvel crossover, if the Living Tribunal from Marvel was involved, it would be extremely powerful. It has near - omnipotent powers and oversees the entire Marvel multiverse. However, in Bleach, Yhwach has extremely powerful abilities as well, like the ability to see the future and rewrite it. So it really depends on the context and how the crossover is set up.
A fun plot could involve the Marvel heroes visiting the Soul Society. Captain America might be intrigued by the strict hierarchical structure and the sense of honor among the Soul Reapers. Meanwhile, Spider - Man could get into all sorts of trouble exploring the different districts of the Soul Society, and his quick - wittedness would be put to the test when dealing with the various spirits and Soul Reapers.
One idea could be that Ichigo Kurosaki from Bleach somehow gets transported to the Victorian era world of Black Butler. He could be mistaken for a demon or some kind of otherworldly being. Sebastian, being the ever - curious demon, might be intrigued by Ichigo's powers and his strong sense of justice. Another idea is that Ciel Phantomhive has to deal with Hollows suddenly appearing in his world. He enlists the help of the Soul Reapers from Bleach to combat this new threat. They could form an uneasy alliance where the Soul Reapers are not used to the complex politics of the Black Butler world, and Ciel has to figure out how to work with these super - powered beings.
I really like 'The Shinigami and the Ninja'. It's a well - written fanfic where the author does a great job of integrating the different mythologies and power systems. You get to see Bleach characters like Rukia in the Naruto world, dealing with things like chakra and ninja villages. It adds a fresh twist to both universes.
One characteristic is the exploration of new settings. Since it's a crossover, Toshiro is often placed in different fictional worlds or situations not in the original Bleach story. For example, he might end up in a steampunk world or a world with magic systems different from Soul Society's.
One interesting idea could be that characters from Bleach are somehow transported to the Naruto world during the Chunin Exams. For example, Ichigo could end up participating in the exams. He might use his Soul Reaper powers in the forest of death, which would give him an edge against the ninja there. His Zanpakuto skills could be misinterpreted as some kind of advanced ninja tool or jutsu.
A good 'Bleach' crossover fanfiction often has well - integrated universes. For example, if it's a 'Bleach' and 'Dragon Ball' crossover, the power levels and the different types of energy in both worlds should be combined in a believable way. If the fanfic just throws the characters together without any thought to how their worlds' rules work, it won't be as good.
One great Hinata crossover with Bleach fanfiction is 'Hinata in the World of Bleach'. It shows Hinata from Naruto ending up in the Bleach universe. She brings her unique skills and gentle nature to the story. It's interesting to see how she interacts with the characters in Bleach like Ichigo and Rukia.