One idea could be that the Jedi or Sith from Star Wars somehow end up in the RWBY universe. Maybe a Sith Lord is after the Dust in RWBY for its power - similar to how they seek the Force in Star Wars. The Jedi could then come to stop the Sith and end up allying with the RWBY characters.
A more character - driven plot could be about a Star Wars character, like Anakin Skywalker, being reincarnated in the RWBY universe. He would have to deal with his past traumas while also adapting to this new world with Huntsmen and Grimm. His relationship with the RWBY characters could be complex, as he tries to find his place in this new reality.
One interesting idea could be the technology exchange. For example, the Star Trek's warp drive might be compared or even combined with the Star Wars' hyperdrive in some creative ways. Another could be the meeting of characters. Imagine Captain Kirk facing off against Darth Vader. It would be a clash of two very different leadership styles and moral compasses.
Well, you could create a story where the Naruto characters are in the Star Wars universe. Picture Naruto as a Jedi in training, learning the ways of the lightsaber and the Force from the likes of Luke Skywalker. Or perhaps Sasuke could be a Sith apprentice, his Sharingan abilities adding a new dimension to Sith powers. And Hinata could be a Jedi healer, using her Byakugan to sense injuries in a different way.
Well, a cool concept would be for Luke Skywalker to end up in the Marvel universe and train some of the young superheroes in the ways of the Force. Or perhaps the X - Men could find themselves in a conflict in the Star Wars galaxy, with their mutant abilities adding a new dynamic to the Star Wars battles.
One idea could be a crossover with the original Star Wars trilogy characters. Maybe the Rebels crew teams up with Luke Skywalker and Han Solo to take on a new, powerful Sith threat. This would bring together the scrappy underdog feel of the Rebels with the classic heroism of the original trilogy.
Another idea is a story where the Covenant from Halo discovers the Force in the Star Wars universe. They might see it as a new form of 'holy' power and try to either harness it or destroy those who possess it. This could lead to some epic battles between the Covenant and the Jedi or Sith.
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 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.
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.