His character might become more empathetic. As a weapon, he would experience being wielded and controlled, which could make him understand Tsubaki's position better. He may develop a new sense of respect for meisters in general.
In a fanfiction where Black Star becomes a weapon, his design as a weapon would probably be very unique. Given his love for being flashy and his ninja - like style, he could be a weapon that has a lot of sharp edges and a sleek, black design. Maybe he'd be a sword with some sort of special ability related to his ninja skills, like being able to disappear and reappear during combat, or having the ability to cut through dimensions.
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.
One idea could be to create an OC weapon that is a scythe with a hidden blade. It could have a unique backstory, like it was forged in a forgotten realm. The character using it could be a bit of a rebel, not fully conforming to the rules of the Death Weapon Meister Academy.
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 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.
One common theme is power and growth. Black Star is often depicted as striving to become more powerful, whether it's mastering new techniques or unlocking hidden powers. Another theme is friendship. His relationships with characters like Maka, Soul, and others are explored, showing how they support and sometimes challenge him on his journey.
Naruto could be seen as a weapon that has the spirit of a ninja within it. His chakra system might be reworked to be more like the soul wavelength in Soul Eater. His Nine - Tailed Fox power could be a hidden, extremely powerful aspect of the weapon. He would likely have a strong sense of justice still, but it would be expressed in the context of the Soul Eater universe, like fighting to protect souls instead of the Hidden Leaf Village.
They have a complex and evolving relationship. Sometimes they're partners, sometimes they have conflicts, but it all adds to the depth of the story.
Black Star leaving could trigger a series of events. He could end up in a place full of dark magic where he has to fight to survive. Meanwhile, back in Death City, there could be an attack by an enemy who was waiting for Black Star to be gone. The remaining characters would have to step up and defend the city without him, which could be a great source of character development.
One great fanfiction is 'The Bond Beyond Souls'. It beautifully explores the dynamic between Black Star and Kid. The author really delves into their different personalities, with Black Star's brashness and Kid's precision. It has some really cool action scenes that are true to the Soul Eater style.