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 plot could involve the Joker finding out about the Grimm and deciding to use them for his own amusement. He could start manipulating the Grimm to attack certain areas, and Team RWBY has to figure out who is behind this new and strange behavior. Or perhaps he uses his mind - games on Ruby, making her question her role as a huntress and the very nature of good and evil in the RWBY world.
Maybe the Joker has a device that can scramble the semblances of the RWBY characters. This causes all sorts of chaos as the characters have to adapt without their normal powers. He could then set up a series of 'games' for them to play in order to get their semblances back, all while he enjoys the mayhem that ensues.
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 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.
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.
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.
One idea could be that the gamer character brings in unique gaming skills like leveling up abilities faster than normal in the RWBY world. This could disrupt the balance of power among the teams and create new rivalries or alliances as others try to figure out this new 'gamer' ability.
One idea could be Deadpool getting dropped into the RWBY universe and thinking it's some sort of crazy new mission. He starts trying to team up with the RWBY girls, but his chaotic nature constantly causes misunderstandings. For example, he might mistake a Grimm for one of his usual foes and go after it in his own wacky way, much to the confusion of the RWBY characters.