A possible plot idea is that Slender Man is a creation of an ancient and dark experiment in the RWBY world that went wrong long ago. Now he has re - emerged and is seeking revenge on those who originally created him. The RWBY team gets involved when they discover that his actions are endangering the safety of the entire world. They have to battle not only his physical form but also the psychological fear he instills in people. Slender Man could also have the ability to manipulate the nightmares of the RWBY characters, which adds a whole new level of horror and psychological thriller elements to the fanfiction.
Well, perhaps there could be a fanfic where Slender Man is actually an ancient evil that was sealed away in the RWBY world long ago. And due to some recent events, like the opening of a certain relic or a powerful magic spell gone wrong, he is released. The RWBY characters then have to figure out his weaknesses by researching old legends. Or it could be that Slender Man has a connection to the White Fang. He uses their anger and desperation to his advantage, secretly manipulating them to do his bidding while the RWBY gang tries to stop this new threat.
One idea could be that Slender Man somehow gets transported into the Simpsons' world. He might start lurking around Springfield Elementary, causing the kids, including Bart and Lisa, to feel an unexplained sense of dread. Another plot could involve Homer mistaking Slender Man for a really tall, creepy new neighbor and trying to befriend him in his typical, bumbling way.
Ruby would probably be both curious and cautious. Her adventurous spirit might make her want to learn more about this strange new entity, but her sense of responsibility for her team would keep her from being too reckless. Weiss would likely be initially dismissive, thinking it's some kind of trick or a new enemy that can be easily defeated with her skills and Dust magic. Blake would be more on guard, her past experiences making her more aware of hidden threats. Yang would probably be the first to want to throw a punch, seeing Slender Man as a direct threat to her and her sister.
Maybe in the fanfiction, Slender Man has an alliance with Orochimaru. Orochimaru is always interested in strange and powerful beings. Together, they plan to take over the ninja world. Naruto and his friends then have to not only face Orochimaru's usual schemes but also deal with the terrifying abilities of Slender Man.
One idea could be that Spider - Man gets transported to the RWBY universe. He has to adapt to the new world with its unique Grimm threats while also trying to find a way back home. Another could be a team - up where the RWBY girls and Spider - Man fight a common enemy that has somehow managed to cross between their universes, like a super - powered criminal who has made a deal with the Grimm.
Another plot could involve a character from RWBY, like Ruby, getting lost in New York City (Spider - Man's turf). Spider - Man discovers her and at first is suspicious. But as they start to communicate, they realize they can help each other. Ruby can show Spider - Man some unique combat moves from her world, and Spider - Man can teach her about the city and how to use her powers more subtly in a world full of normal humans.
Another plot could involve a RWBY character ending up in the Naruto universe. Say, Weiss gets sent to Konoha. She might initially struggle with the different culture and the use of chakra, but eventually she could use her Glyphs in unique ways by combining them with chakra. This could also lead to some interesting interactions between her and the ninja students there.
A cool plot could involve the characters from Frozen getting stuck in the RWBY world and having to learn how to fight Grimm. Anna might be really eager to learn combat skills, while Elsa is more focused on controlling her powers in this new and dangerous environment.
One idea could be that Gilgamesh, with his vast power and arrogance, arrives in the RWBY world. He might initially see the Huntsmen and Huntresses as beneath him but then becomes intrigued by Ruby's unique silver - eyed power. Another could be a scenario where Gilgamesh allies with a villain in RWBY, like Cinder, and together they try to reshape Remnant according to their desires.
One idea could be that Iron Man's technology somehow gets integrated into the world of RWBY. Maybe his suits' energy systems could be used to power up the Huntsmen and Huntresses' weapons. For example, Ruby could have a Crescent Rose upgrade with Iron Man's repulsor technology, allowing for more powerful ranged attacks.