A great idea for a JJBA Bleach crossover fanfic could be centered around the concept of alternate dimensions. Since both series deal with different planes of existence in their own ways. The characters could accidentally get transported to the other universe and have to find their way back while getting involved in the local conflicts. For instance, Josuke Higashikata could end up in the Soul Society and use his Crazy Diamond to fix things (both literally and figuratively) while befriending some of the Soul Reapers. And some of the Bleach characters could find themselves in Morioh and have to deal with the strange Stand - related incidents there.
Well, you could focus on the character crossovers. Imagine Jotaro Kujo meeting Ichigo Kurosaki. Jotaro's cool and collected demeanor would contrast with Ichigo's more hot - headed nature. They could learn from each other's fighting styles. And then there's the possibility of Stand users having to adapt to the spiritual energy in the Bleach world and vice versa. This could lead to some really interesting power - up and training sequences for the characters involved.
One idea could be a crossover with another popular anime like Naruto. For example, Jotaro could meet Naruto and they could have a battle of wills and powers. Naruto's positive attitude might clash with Jotaro's more stoic nature.
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 crossover with One Piece might also work well. Joseph Joestar could find himself on the Grand Line, interacting with the Straw Hat Pirates. His hamon abilities could be a strange and new power for the pirates to encounter, and he could help them out in battles against the likes of the World Government or other powerful foes, while also learning about the Devil Fruits and Haki.
One idea could be a scenario where Naruto and Ichigo meet in a dimension between their worlds. They initially misunderstand each other due to their different combat styles and powers. Naruto might mistake Ichigo's Zanpakuto for a strange kunai at first. But then they realize they are both protectors of their respective realms and team up to face a common threat that is endangering both the Naruto and Bleach universes, like an ancient evil entity that feeds on spiritual and chakra energy.
One idea could be the Persona characters getting sucked into the JJBA universe. For example, the Phantom Thieves could end up in Morioh and have to deal with Stand users while trying to find a way back home. They might team up with Josuke and his friends to fight a common enemy that's causing chaos in both their worlds in a strange way.
First, you should do your research on both JJBA and Bleach. Know the characters, their powers, and the rules of their respective universes. Then, come up with a basic plot idea, like a conflict that involves both universes. For example, a villain who wants to merge the two worlds for his own evil purposes. After that, start thinking about which characters you want to feature and how they will interact.
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 and Maka meet in a dimension that is a blend of their two worlds. They initially mistake each other for enemies but then realize they have a common goal of fighting against a new type of evil that is emerging due to the merging of their universes. Another plot could involve the Soul Reapers and the Meisters exchanging their powers for a short time to better understand each other's fighting styles and learn new techniques. A third plot idea is that the Hollows in Bleach start to target the souls in the Soul Eater world, and the characters from both series must team up to stop this invasion.
One idea could be to explore an alternate timeline where Ichigo never became a Soul Reaper. Maybe he lives a normal life, but strange spiritual events still occur around him, drawing him into the world of the Hollows and Soul Reapers in a different way. Another idea is to focus on a side character, like Uryu, and tell a story from his perspective during a major arc that we didn't see much of in the original series. For example, his experiences during the Arrancar arc when he was off on his own missions.
Perhaps there is a crossover of the worlds where the Hollows start attacking the One Piece islands because they sense a large amount of spiritual energy there. The Soul Reapers follow them and have to work with the local pirates, including the Straw Hats, to defend the islands. In the process, characters from both series learn about each other's powers and cultures. Luffy might be intrigued by Ichigo's ability to transform into different forms, and Ichigo could be amazed by Luffy's stretching powers.