Maybe the Persona characters end up in Remnant. They could help the RWBY teams with their unique Persona abilities. The Velvet Room could also exist in Remnant in some form, and Ozpin might have a connection to it. The Persona users could teach the RWBY characters about the power of the mind and how to use it in combat.
One idea could be that the RWBY characters enter the Metaverse from Persona. They might have to face their Shadows, which could be related to their Grimm - fighting fears or personal insecurities. For example, Ruby could face a Shadow that represents her fear of not being able to protect everyone.
Another plot could involve a shared enemy that is a combination of a Grimm and a Persona - like entity. The RWBY and Persona characters would have to combine their skills and knowledge. The Persona characters could introduce the concept of Social Links to the RWBY world, and through building relationships, they all gain new powers to fight this strange new foe.
Maybe the Persona users' powers start to malfunction in the RWBY world due to the different nature of the place. So they have to figure out how to control their powers again while also helping the RWBY characters in their battles. The RWBY characters could be intrigued by the Persona powers and try to learn from them.
A great plot could involve the transfer of Personas between the two groups. For example, a character from Persona 3 accidentally gets a Persona that belongs in the world of Persona 4 and vice versa. This causes chaos as they have to learn to control and understand these new entities while also dealing with the reactions of their respective teams.
One idea could be to cross it with another popular RPG. For example, having the Persona 4 characters enter the world of Final Fantasy. They could interact with the different races and magic systems there. Another could be a crossover with a detective - themed anime. The Investigation Team could team up with the detectives from that anime to solve a mystery that spans both worlds.
A cool plot could involve the Persona 5 crew going back in time to the events of Persona 3. They might end up interfering with some of the original events, which could have unforeseen consequences. For example, they could accidentally change the way a key character in Persona 3 awakens their Persona, and then have to figure out how to fix it while also dealing with the Shadows in that era.
Another option is a crossover with the Pokémon world. The Phantom Thieves could travel to a region in the Pokémon world and discover that someone is using Persona powers to make Pokémon act out of control. They would need to catch Pokémon themselves and use their combined powers of Personas and Pokémon to stop the bad guy. It would be interesting to see how they interact with Pokémon trainers and the whole Pokémon ecosystem.
One idea could be the characters from Persona 4 and Persona 5 swapping their worlds. So the Phantom Thieves end up in Inaba and have to deal with the Midnight Channel mystery. And the Investigation Team finds themselves in Tokyo and gets involved in the Metaverse. It would be really cool to see how they interact with the different environments and characters.
A great plot could be that a new threat emerges that has the power to erase the memories of people in both Persona 4 and Persona 5's worlds. The characters from both games must work together to find the source of this threat. The Phantom Thieves could use their infiltration skills, while the Investigation Team could use their detective - like skills to gather clues. As they progress, they discover that the root of the problem lies in an ancient evil that was once sealed away but has now been awakened due to some modern - day events.
One possible plot could be that the characters from RWBY end up in the Persona universe and have to learn how to use Personas to fight the Shadows. For example, Ruby could discover her Persona is related to her speed and scythe skills, and she uses it to quickly dispatch enemies. The team would also have to deal with the social aspects of the Persona world, like building relationships to strengthen their Personas.
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.
One idea could be that the RWBY characters find themselves in the Fallout wasteland. Ruby could use her Crescent Rose to fend off mutants. Weiss might struggle with the harsh and dirty environment compared to her usual clean Atlesian surroundings. Blake could use her stealth skills to navigate dangerous areas, and Yang's strength would be useful in combat against raiders. They could be on a quest to find a way back to their own world while also trying to make the wasteland a better place.