Maybe the cat is actually a transformed character. In RWBY fanfiction, it could be a person who was cursed and turned into a cat. The RWBY team stumbles upon the cat and gradually discovers its true identity. They then have to find a way to break the curse, which could involve going on a quest to find a magical artifact or meeting a powerful mage.
The man could be a detective. He is on a difficult case. While he is staking out a suspect's house, he notices a cat that seems to be acting strangely. The cat leads him to a clue that he would have otherwise missed. The man then forms a connection with the cat, and as he solves the case, the cat becomes a symbol of his good luck. After the case is over, he takes the cat home with him.
In a 'doomguy in rwby fanfiction', Doomguy could be a mysterious figure that the RWBY characters hear rumors about. He's said to be this super - powerful warrior from another realm. Eventually, they cross paths. Doomguy has a hard time adjusting to the more colorful and magical world of RWBY at first. But as he gets used to it, he starts using the resources of this new world, like Dust, to enhance his weapons. His presence also changes the dynamics of the relationships between the RWBY characters as they all learn from his no - nonsense attitude towards fighting.
The man could be a lonely artist. His cat is his only companion. One day, he meets two women at an art exhibition. One woman is an art critic and the other is an artist like him. The cat takes a liking to the art critic woman. This makes the man start to interact more with her. Meanwhile, the other woman, the fellow artist, starts to feel a bit jealous. But in the end, they all realize that they can be friends and support each other's art, with the cat being a cute addition to their group.
One possible plot could be that Ulquiorra, with his cold and emotionless nature, is suddenly thrust into the world of RWBY. He might initially be confused by the colorful and energetic world filled with Huntsmen and Huntresses. Maybe he gets involved in a battle against the Grimm and his unique Hollow powers create a stir among the RWBY characters.
One possible interesting plot could be that the RWBY characters are transported into the Pokemon world and have to learn how to catch and train Pokemon in order to find a way back home. They might struggle with the different battle styles compared to their usual combat in Remnant.
A plot could involve a mad scientist in the RWBY world who tries to recreate the power of Godzilla using Dust. However, the experiment goes wrong and starts to attract the real Godzilla. The RWBY teams then have to deal with the chaos caused by the approaching Godzilla. They might think that Godzilla is there to destroy their world, but as they fight, they discover that Godzilla is actually attracted to the unstable energy that the mad scientist created, which is also a threat to the balance of their world. So, they have to find a way to work with Godzilla to neutralize the dangerous energy source.
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 cool plot might involve a dark force that is affecting both the RWBY world and Kirby's world. This force is causing strange things to happen, like Grimm starting to appear in Dream Land and some of Kirby's enemies showing up in Remnant. Ruby, Weiss, Blake, and Yang meet Kirby, and they initially don't understand him but soon realize his power. They work together to find the source of this dark force. Kirby uses his inhale ability to help them gather information by sucking up some of the smaller Grimm and enemies to learn more about what's going on.
The plot could revolve around a new character who arrives in Remnant with a Chaos Emerald. This character is being chased by an unknown force from another dimension. The RWBY team decides to help the new character protect the emerald. As they do, they learn about the different abilities of the emerald. For example, it can open portals to other dimensions. They have to fight off the pursuers while also dealing with the temptation of using the emerald's power for their own gain.
Maybe the fanfiction could revolve around a tournament - like scenario within the Battlefield 1 setting. The RWBY teams are pitted against various factions from Battlefield 1. They need to score points by achieving certain objectives, like capturing flags or holding strategic positions. The RWBY characters' unique abilities would add an interesting twist to the traditional Battlefield 1 gameplay. For example, Ruby's scythe could be a powerful melee weapon against the bayonet - wielding soldiers.