The interactions would also depend on the setting of the crossover. If it's set in the world of Black Butler, the Soul Eater characters would have to adapt to the Victorian - era society. Maka, for instance, might be frustrated by the strict social norms while trying to hunt for kishin eggs. The Black Butler characters would be in charge and would likely view the Soul Eater characters as outsiders at first. However, as they work together against common threats, they would start to understand and respect each other. If it's set in Soul Eater's Death City, the Black Butler characters would be in a more modern and chaotic environment, which would be a big change for them.
Perhaps there could be a plot where the two main characters, Ciel and Maka, swap their partners. So Maka has to deal with Sebastian's sly and demonic ways while trying to achieve her goals in the Black Butler world. And Ciel has to work with Soul, a weapon with his own personality. This would force both characters to adapt to new situations and learn from each other's worlds.
Perhaps in this crossover, the concept of the tailed beasts in Naruto are related to the Kishin in Soul Eater. The tailed beasts are seen as powerful and dangerous entities, much like the Kishin. A group of characters from both universes, including Naruto, Maka, and Soul, come together to find a way to either control or destroy these powerful forces that threaten both their worlds. They would have to learn from each other's strengths and weaknesses to succeed.
In many Black Star fanfictions, his relationship with the other characters is a key aspect. He might have interesting interactions with Maka and Soul. Maybe there's a fanfic where they team up for a special mission and Black Star has to learn to work with them in a different way than he's used to.
There could be a really interesting fanfiction where Naruto characters possess immortal souls like in Soul Eater. For example, Naruto's strong will could be translated into an extremely powerful and unbreakable immortal soul. The ninja world's chakra system might interact with the concept of souls from Soul Eater in unique ways, like using chakra to enhance or protect the soul. Maybe Sasuke's Sharingan could have a new ability related to seeing and manipulating the immortal souls.
One idea could be to have the characters from BNHA attend the Death Weapon Meister Academy in the Soul Eater universe. They could learn about soul resonance and how to handle weapons in a whole new way. For example, Deku could be paired with a unique weapon and struggle to master the new techniques.
Next, when writing the crossover, focus on how the characters would react to the new environment. If Bakugo were to enter the Soul Eater world, he'd probably be very frustrated at first with all the new rules. You should also make sure to balance the elements from both series. Don't over - emphasize one over the other. For example, give equal importance to the development of BNHA - style quirks and Soul Eater - style soul abilities.
One idea could be that Black Star leaves to pursue a solo mission to prove his strength without relying on his friends. He might encounter a powerful and mysterious foe that challenges his very core beliefs. This would make for an exciting story as he battles not just physically but also internally.
The theme of friendship also shows up a lot. Despite his brash exterior, Black Star has friends. Fanfictions may show how his friendships are tested, for example, when he has to make a difficult choice that could potentially harm a friend, and how he navigates through that moral dilemma.
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.