One idea could be Cinder being transported to an alternate universe within the RWBY world where the roles are reversed. For example, she could find herself in a world where she is a huntress in training, facing the same challenges as the main characters but from a different perspective. Another plot could involve a team - up between Cinder and a group of RWBY characters against a common, greater enemy that threatens both their goals and the world. Maybe a new, powerful Grimm that has abilities they've never seen before.
A more emotional plot could be that they discover they have a shared past that they were unaware of. Maybe they both had a connection to a certain character who was important in their lives before they became who they are now. This revelation makes them question their own paths and they start to see each other in a new light, leading to a tentative alliance and exploration of their similarities and differences.
One idea could be a crossover where Cinder somehow gets transported to the Naruto universe. She might initially try to use her powers to gain control, but then realizes she has to adapt to the new chakra - based powers. Meanwhile, RWBY characters could also end up in the Naruto world in search of Cinder and end up forming unexpected alliances with Naruto and his friends.
A possible plot is that a portal opens between the Doom universe and RWBY's. Some of the Doom demons start spilling into Remnant, the world of RWBY. Ruby and her team initially mistake them for a new, extremely powerful type of Grimm. As they try to fight them, they realize they need different strategies. Meanwhile, the Doom Slayer follows the demons through the portal to stop them. Eventually, the two groups meet and learn from each other's combat styles to defeat the invading demons.
Well, a cool plot for a Witcher RWBY crossover fanfiction might involve the Witcher's magic and potions being introduced to the RWBY universe. For example, a potion that gives the RWBY characters temporary enhanced senses like a Witcher. Or perhaps a situation where a powerful Witcher villain comes to the RWBY world and the teams have to combine their skills to defeat him. The different fighting styles of the Witcher and RWBY characters would make for some really exciting battle scenes.
Well, another plot could involve Sans and Papyrus coming to the RWBY world. Papyrus might be fascinated by the Grimm and try to befriend them, while Sans could use his powers to help the RWBY team in their battles against Salem. He could also share his knowledge of timelines with Ozpin.
One idea could be the Overwatch heroes being transported to the RWBY world. They could team up with Team RWBY to fight a new common enemy, like a powerful Grimm that is immune to normal attacks but vulnerable to Overwatch tech. Another plot could involve a character exchange, where a RWBY character like Ruby goes to the Overwatch world and learns about their technology and fighting styles, and then brings back new knowledge to her own world.
One idea could be that the Dragonborn is somehow transported to the RWBY world. They could use their shouts to combat the Grimm. For example, 'Fus Ro Dah' could send Grimm flying back, and the local Huntsmen and Huntresses are both amazed and a bit wary of this new power.
One interesting plot idea could be that the Joker somehow gets transported to the RWBY universe. He might start causing chaos in Vale, tricking the villains and heroes alike. For example, he could convince Roman Torchwick to do his bidding in a more extreme way, which would then draw the attention of Team RWBY. Another idea is that he teams up with Cinder Fall, but his chaotic nature eventually causes their alliance to crumble as he doesn't follow her grand plans in the way she expects.
A more complex plot could be centered around the concept of the Soul Society in the RWBY world. What if the RWBY characters find out about the afterlife as it exists in Bleach? They might try to find a way to communicate with their fallen comrades who are in the Soul Society. This could open up a whole new dimension of character development and exploration of themes like life, death, and the afterlife.
One idea could be that the Titans from Titanfall end up in the world of RWBY. They could team up with the Huntsmen and Huntresses to fight a new, powerful Grimm threat that is too much for either side to handle alone. The Pilots could share their advanced combat skills with the RWBY characters, and in return, the RWBY characters could teach the Pilots about the use of Aura and Semblances.