One idea could be a crossover with another popular fighting game series like Street Fighter. Characters from both universes could meet in a strange dimension and have to battle it out to find a way home. For example, Ragna from BlazBlue and Ryu from Street Fighter could have an epic showdown.
Another idea is a crossover with a science - fiction universe like Mass Effect. The BlazBlue characters could find themselves on a spaceship in the Mass Effect universe. They would have to deal with things like alien races, advanced technology, and intergalactic politics. Jin Kisaragi might have some interesting interactions with Commander Shepard, as they both have a sense of duty and honor in their own ways.
A BlazBlue crossover fanfic could involve characters from BlazBlue entering a world of high fantasy. Imagine Noel Vermillion in a world full of elves and wizards. She could use her skills in a completely different way, like using her gunblade to fight off magical beasts. It would be interesting to see how the BlazBlue characters adapt to a new and unfamiliar world.
One idea could be to explore how Susanoo's powers interact with the BlazBlue universe's unique combat system. For example, how would his abilities fare against characters like Ragna or Jin? Another idea is to create a story where a character from BlazBlue somehow gains the power of Susanoo, leading to a new power struggle within the game's world. You could also have a crossover where the worlds of BlazBlue and the source of Susanoo (if applicable) start to merge, forcing characters from both to team up or fight against each other to save their respective existences.
For a BlazBlue Hazama crossover fanfiction, a cool concept would be to have Hazama enter a dream - like realm within the BlazBlue world. In this realm, he encounters the deepest fears and desires of the other characters. He could either use this knowledge to further his own dark plans or, surprisingly, be affected by the purity of some of the dreams he witnesses. This could lead to a character arc where he has to question his own motives and actions in the context of the BlazBlue universe.
Maybe in the fanfiction, the Dust from RWBY and the Ars Magus from BlazBlue could be related in some way. The characters could discover that the two energy sources have a common origin, and this knowledge could either be a great power or a great danger. Weiss could be intrigued by the complex magic system of BlazBlue and try to find similarities with her own Glyphs, while characters from BlazBlue might be fascinated by the unique abilities of the RWBY characters.
Perhaps a fanfiction where Terumi loses his powers and has to rely on his wits and the help of some unlikely allies. He could start to see the world from a different perspective without his powers, leading to character growth.
One idea could be that the main character, a new fighter in the BlazBlue universe, suddenly finds himself surrounded by various female characters who are all intrigued by his unique fighting style. They start to vie for his attention, leading to comical and heartwarming situations.
Maybe a fanfic where Jubei takes on a young apprentice. This could show his mentoring side and how he imparts his knowledge and skills. You could include scenes of training, as well as the apprentice getting into trouble and Jubei having to rescue them, all while teaching important life lessons.
A plot could revolve around Rachel's past. Maybe she discovers something about her family or her own origin that she didn't know before. This could lead to a journey of self - discovery. Or perhaps she has to deal with a threat to her kingdom that requires her to use both her wits and powers. For example, an ancient evil awakens and she has to gather allies and find a way to defeat it while also protecting her people.
One idea could be a crossover where the characters from RWBY and Blazblue team up to fight a common enemy. Maybe they have to combine their unique abilities like the Dust powers from RWBY and the Ars Magus from Blazblue.
You could do a crossover within different fable universes. For instance, take the moral - driven characters from Aesop's fables and put them in the world of Native American fables. The animals in Aesop's fables might learn new values from the Native American fable characters. Also, consider mixing fables from different cultures, like Asian fables and European fables, in a fanfic.
Maybe a crossover with a fantasy world where the Darkness entities interact with magical creatures. For example, they could form alliances or rivalries with elves or wizards, which would lead to a whole new set of adventures and conflicts.