One idea could be that the Slenderman somehow enters the RWBY universe during a Grimm attack. The characters of RWBY initially mistake him for a new type of Grimm. But as they start to investigate, they realize he has very different and more sinister powers compared to the Grimm.
One interesting idea could be a crossover of different timelines within RWBY. For example, characters from the past meet the current ones and have to deal with the differences in their worlds. Another could be a crossover of different teams in unexpected ways, like Team RWBY having to team up with a rival team to face a common enemy that is more powerful than any they've faced before. And sometimes, there are crossovers that involve alternate versions of the characters, where their personalities or abilities are slightly different, which can lead to very unique and engaging storylines.
One idea could be that Slenderman and Enderman are from different dimensions but are somehow connected by a mysterious force. They start to communicate telepathically and form an alliance to protect their shared secrets from other mobs in the Minecraft - like world. Another plot could involve Slenderman being lost in Enderman's realm and Enderman helping him find his way back, while they both learn about each other's powers along the way.
One idea could be a crossover where the Slenderman becomes a new killer in Dead by Daylight. He could use his teleportation and mind - controlling abilities to hunt down survivors. For example, he might make survivors hallucinate the location of generators or exits, leading them astray.
Maybe they start as rivals, constantly trying to outdo each other in terms of scaring mortals. But then they realize that together they can create a much more terrifying world. So they start working together, with Zalgo spreading madness and Slenderman preying on the fearful minds.
One idea could be that Jeff the Killer becomes an apprentice of Slenderman. He learns the more supernatural and strategic ways of hunting from Slenderman. Another could be a story where they have to team up against a common enemy, like a powerful group of demon hunters who are after both of them. Or perhaps a plot where Jeff the Killer starts to question his own murderous ways after spending time with Slenderman, who has a more calculating and long - term approach to his actions.
One interesting idea could be the exploration of their contrasting personalities. Slenderman is often depicted as this mysterious, otherworldly being, while Jeff the Killer has his own brand of insanity. In the fanfiction, they might interact in ways that bring out new aspects of each other's characters, like how Slenderman's calmness could either soothe or further enrage Jeff.
One idea could be that Slenderman somehow enters the wizarding world of Harry Potter. Maybe he's drawn by the powerful magic there. He could start causing strange disappearances in the Forbidden Forest, and Harry and his friends have to figure out how to stop this new, otherworldly threat using both their magic skills and their knowledge of the wizarding world's secrets.
One idea could be to have Gilgamesh as a powerful ancient being who awakens in the RWBY world. He could initially be seen as a threat but then form an unlikely alliance with the main characters. For example, he might offer his knowledge of ancient magic to help them fight against a new, powerful Grimm.
One interesting idea could be the redistribution of resources in the RWBY universe. For example, Dust, which is a crucial resource, might be shared more equally among the kingdoms. This could lead to a more balanced power structure and less inequality between the rich and the poor. Another aspect could be the role of the Huntsmen and Huntresses. In a communist - inspired RWBY world, they might work more for the collective good rather than for individual glory or profit. They could be more focused on protecting all people equally, regardless of their social status or wealth.
An OC in RWBY fanfiction could be a character who is a master of stealth. They could use their skills to gather intelligence for the teams at Beacon Academy. Their backstory could involve growing up in a family of thieves, but they decided to use their abilities for good. They might have a Semblance that allows them to become invisible for short periods of time, but it has a drawback like draining their Aura quickly. This OC could interact with characters like Blake, who also has some stealth - like abilities, and they could learn from each other.