A really cool fanfiction idea could be a story that delves into the backstories of the weapons in both series. In RWBY, weapons are an important part of a hunter's identity, and in Soul Eater, the weapons are characters themselves. A fanfiction could explore how these two concepts are related, perhaps with a character from RWBY finding out that their weapon has a hidden connection to the weapon - characters in Soul Eater.
Well, a cool concept could be the exchange of powers. Maybe Weiss could learn some of Soul Eater's magic - based combat styles and incorporate them into her Glyphs. And for Soul Eater characters, they could gain some knowledge about Dust from the RWBY world. Also, a plot where the two worlds start to merge slowly due to some mysterious force, forcing the characters to adapt and find a way to separate the worlds again.
The theme of friendship and teamwork also runs through many of these fanfictions. Just as in the original series, characters from RWBY and Soul Eater come together, and the fanfictions show how they build relationships, trust each other, and work towards common goals. This can include things like facing powerful enemies together or solving problems in the combined universes.
One idea could be a fanfiction where the characters are transported to a modern-day world and have to adapt. For example, Maka and Soul having to go to a normal high school and deal with things like exams and social cliques while still maintaining their weapon - meister bond.
One interesting Soul Eater Soma fanfiction could be about Soma's adventures in a different dimension within the Soul Eater universe. Maybe he teams up with some unlikely allies and battles new and strange enemies.
Another type of Soul Eater reader fanfiction involves the reader being from a different dimension. They get pulled into the Soul Eater world and have to adapt. This can lead to really cool crossovers in the story, like bringing in elements from the reader's home dimension and seeing how they clash or blend with the Soul Eater universe.
One interesting fanfiction could be about Death the Girl's adventures in a parallel world where the rules of the Soul Eater universe are slightly different. Maybe she has to team up with unlikely allies to face a new kind of threat that endangers not just the DWMA but the entire world of the living and the dead.
There's 'RWBY and Soul Eater: A United Front'. This fanfiction focuses on how the teams from RWBY and the characters from Soul Eater would team up to face a common enemy. It's full of action and has some really cool fight scenes that combine the fighting styles of both franchises.
One idea could be that Soul's sickness is caused by an unbalanced resonance with Maka. Maybe they've been overusing their Soul Resonance in battles and it's taking a toll on Soul. Another idea is that an enemy has cursed Soul, making him sick in a way that weakens his combat abilities and his connection to his weapon form. For example, the curse could make it painful for him to transform.
Some good Soul Eater fanfictions are those that expand on the world of the DWMA. They could describe new types of weapons and meisters that exist in other parts of the world. A particular fanfiction I like is one where the characters are transported to a modern - day Earth and have to adapt to a completely different lifestyle while still having their powers. It's really fun to see how they interact with normal humans and deal with things like technology.
One interesting concept could be a crossover with another popular anime like Naruto. Imagine Soul Eater characters in the ninja world, with their unique weapons and abilities. Another concept might be a crossover with a fantasy series like Lord of the Rings, where the Soul Eater characters have to face the dark forces in Middle - earth. And a crossover with a superhero universe could also be exciting, seeing how they would interact with superheroes and villains.