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.
One possible plot could be that Mario and Kirby's worlds are somehow merged due to a powerful villain's spell. They have to team up to find the source of the spell and break it. Along the way, Mario could use his jumping and power - up skills while Kirby uses his ability to copy enemies' powers. They might encounter challenges like a new type of Goomba that can fly like some of Kirby's foes.
One possible plot could be that an evil force is threatening both Kirby's and Sonic's worlds. They accidentally cross paths while investigating strange happenings. Kirby can use his ability to copy powers and Sonic can use his speed to explore different areas. Together, they combine their skills to defeat the common enemy. For example, Kirby might copy Sonic's speed ability for a short time, allowing him to quickly reach parts of the battlefield that Sonic couldn't access alone. And Sonic could use Kirby's star power to enhance his own attacks.
One interesting concept could be the combination of Kirby's unique powers, like inhaling enemies and stealing their abilities, with the combat styles and semblances in RWBY. For example, Kirby might gain Ruby's speed semblance and use it in a creative way during battles in the RWBY world.
One idea could be Kirby arriving in Remnant and being mistaken for a new type of Grimm at first. Then, as the characters get to know Kirby, they realize his unique abilities can be a great asset in battles against Salem's forces.
One possible plot could be that Doflamingo somehow ends up in the RWBY world. He might initially try to take over, using his strings to manipulate the Grimm. But the RWBY team discovers his true nature and battles him. Ruby could use her speed to avoid his string attacks while Weiss uses her glyphs to counter his powers.
Another plot could be that an evil force in the RWBY universe discovers the existence of the Omnitrix from another dimension. They manage to create a device to pull Ben and the Omnitrix into their world. Ben has to adapt quickly to this new world while Team RWBY, who are initially suspicious of him, realizes that he could be an ally against this new threat. They join forces, with Ben using his alien forms in creative ways to combat the Grimm and the evil force's minions.
A possible plot is that the RWBY characters end up in New York. Ruby, with her speed and scythe skills, is initially mistaken for a threat by Spiderman. But when they realize they are both on the side of good, they join forces. Weiss could use her glyphs in new and creative ways in the urban environment, Blake's stealth and acrobatic skills could be put to great use, and Yang's strength would be a valuable asset. Together they take on a criminal organization that has somehow gotten hold of some Dust and is using it for evil purposes.
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.
One possible plot could be that the Autobots and Decepticons end up on Remnant. The RWBY teams initially mistake them for new types of Grimm. But then they realize they are sentient machines. They team up with the Autobots to fight off the Decepticons who are trying to mine Remnant for its resources.