Jin could also be a great crossover character with Terumi. Jin's ice powers contrast with Terumi's dark powers. They could be pitted against a common enemy that requires them to combine their skills, which would be interesting to explore as they have different motives.
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.
First, do some research on both BlazBlue and Susanoo. Know their lore, characters, and power systems well. Then, come up with a basic plot idea, like how the two worlds will collide or how a character from one will enter the other. You can start with a simple scenario, such as a BlazBlue character stumbling upon a strange portal that leads to Susanoo's world.
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 character that might not be thought of immediately but could be a great opponent for Hazama is Kokonoe. She is extremely intelligent and has access to a lot of technology and power within the BlazBlue world. Her scientific knowledge could potentially counter Hazama's otherworldly and mysterious powers. She could also outwit him in a battle of wits, which would be very engaging in a fanfiction.
One common theme is probably the clash of different fighting styles. Taokaka has her own unique way of fighting, and in the BlazBlue universe, there are diverse combat styles too. So, a lot of fanfics might explore how these different styles interact during battles.
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.
Ruby and Noel would be a great pairing. Ruby is young, energetic, and has a strong sense of justice. Noel is also a very determined and kind - hearted character. They could bond over their shared values and also learn from each other's combat styles. Ruby could show Noel some of her acrobatic moves with Crescent Rose, while Noel could teach Ruby some of the finer points of her swordsmanship.