A possible plot could be that there is a strange energy disturbance that starts to merge the three universes together. Characters like Ragna from Blazblue, the Persona users, and Team RWBY find themselves in a strange new world that is a mix of all three. They must figure out what is causing this and how to stop it. Maybe there are also new enemies that are created from the combination of the different mythologies and technologies of the three universes. For instance, Grimm with the ability to use Persona - like powers or Blazblue - style Ars Magus. It would be a great opportunity for character development as they learn to work together despite their differences.