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.
A possible plot is that a portal opens between the RWBY world and the Fallout world due to some sort of experimental accident in both universes. RWBY characters start to appear in different parts of the wasteland. Some local Fallout survivors think they are some new kind of mutant at first. But as they see the RWBY characters' abilities and intentions, they start to work together. The RWBY team is searching for a way to close the portal to prevent any more chaos from spilling over into either world.
One idea could be that the characters from RWBY end up in the Fallout wasteland. Ruby could use her scythe skills to fight off raiders and mutants. Weiss might use her glyphs to protect the group from radiation in certain areas. Another idea is that the Dust from RWBY could be a valuable resource in the Fallout world, leading to various power struggles.
One idea could be that Naruto characters are transported to the RWBY world. Naruto himself could befriend Ruby and they could learn from each other's combat styles. For example, Naruto's chakra control might inspire Ruby to better control her own energy. Another plot could involve the Akatsuki from Naruto being interested in the Dust in RWBY's world and causing chaos to obtain it.
One idea could be that the Tenno from Warframe are sent to the RWBY universe due to a trans - dimensional rift. They team up with the RWBY characters to fight a new common enemy that is a fusion of the Grineer and Grimm. The Tenno can introduce their advanced technology and combat skills, while the RWBY characters show their unique Semblances and teamwork.
One idea could be that the Servants from Fate end up in the RWBY world and have to team up with the RWBY characters to fight a new common enemy. For example, a powerful Grimm that has some sort of magic ability similar to a Servant's Noble Phantasm. Another plot could involve the RWBY characters being summoned into the Fate universe and having to participate in the Holy Grail War, but with their own unique skills and abilities that disrupt the normal flow of the war. A third idea might be that a mysterious force starts fusing the two worlds together, and the characters from both have to figure out how to stop it while also dealing with the cultural and power differences between them.
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.
A plot could involve a group of settlers from Fallout 3 trying to make a new start in the Mojave. They run into trouble with the various factions in New Vegas. Meanwhile, a character from New Vegas goes to the Capital Wasteland to learn about their different ways of survival. This leads to an exchange of ideas and technology between the two groups, as well as some clashes due to different values.
One idea could be that the Dragonborn from Skyrim somehow gets transported to the post - apocalyptic world of Fallout 4. Maybe they have to deal with the new threats like the raiders and mutants using their dragon - slaying skills. Another plot could involve the Sole Survivor from Fallout 4 finding a way into Skyrim and getting caught up in the civil war there, having to choose a side while also trying to find a way back home. Or perhaps a magical artifact from Skyrim ends up in Fallout 4 and causes all kinds of chaos as different factions fight over it.
One idea could be a character from Fallout finding themselves in Skyrim. Maybe they bring their advanced weaponry, like a laser pistol, into Skyrim's medieval - like world. This would cause quite a stir among the locals, who have never seen such technology. The character could then get involved in Skyrim's political intrigue, using their unique skills to help or hinder different factions. Another plot could involve a Skyrim mage traveling to the Fallout wasteland. They would be amazed by the strange, post - apocalyptic magic (technology) there and try to learn about it, while also using their own magic to survive against the mutated creatures.
One idea could be a crossover where characters from Fallout enter the RWBY universe. For example, a Vault Dweller could end up in Vale and have to adapt to the Grimm - filled world while also trying to find a way back home. Another could be a story where RWBY characters are thrown into a post - apocalyptic setting like in Fallout, dealing with radiation and mutated creatures.
Perhaps there could be a situation where the Grimm from RWBY start to invade the world of Genlock. The Genlock team, with their diverse skills and abilities, would have to learn about the Grimm from the RWBY characters in order to fight them effectively. And in return, the RWBY characters could learn about Genlock's technology and combat strategies. For example, they could figure out how to use the Genlock's mech - like suits in their battles against the Grimm. It would be a great opportunity for character development as they interact and adapt to each other's ways.