One idea could be that the Horsemen from DarkSiders end up in the world of RWBY. Maybe War arrives first and is mistaken for a powerful Grimm. He then has to figure out how to interact with the Huntsmen and Huntresses while also trying to find a way back to his own realm. Another idea is that the characters from RWBY somehow stumble into the post - apocalyptic world of DarkSiders and have to adapt to the harsher and more dangerous environment.
One idea could be that Naruto is somehow transported to the Darksiders universe. He might initially be confused by the dark and desolate landscape. But with his ninja skills and determination, he could start to explore and try to find a way back home. He could encounter the Horsemen and at first be seen as an enemy, but later they realize he has no malicious intent.
Another possibility is a crossover with 'God of War'. Death and Kratos could meet. Given their both powerful and somewhat vengeful natures, they could either become bitter enemies or reluctant allies. The story could explore their different views on death, with Death being an embodiment of it and Kratos having a very personal and violent relationship with death in his own journey.
Another interesting concept for a Darksiders War crossover fanfiction could be a crossover with the Diablo series. Imagine the dark and gritty world of Diablo mixing with the post - apocalyptic world of Darksiders. The heroes from Darksiders could be lured into the world of Sanctuary in Diablo, where they have to deal with the hordes of demons while also trying to find a way back home.
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.
The characters could start off as enemies. The Horsemen might be seen as threats by the RWBY characters because of their intimidating appearance and their connection to destruction in the case of War. But then they could discover a common goal, such as stopping a powerful entity that is causing chaos in both worlds. They would then have to learn to work together, with the RWBY characters teaching the Horsemen about their world and vice versa.
One idea could be that Superman arrives in the RWBY world and has to learn about Dust and its powers. He could team up with Team RWBY to fight a common enemy, like a powerful Grimm that is immune to normal attacks but not to Superman's strength. Another idea is that Ruby discovers she has some Kryptonian-like abilities and Superman helps her understand and control them.
One idea could be the Doom Slayer arriving in the RWBY world. He could be on a mission to stop some sort of demonic invasion that has seeped into Remnant. His powerful arsenal and unwavering determination would be a huge contrast to the Huntsmen and Huntresses of RWBY.
Another interesting concept is that some of the Titans could be modeled after the RWBY characters' weapons or fighting styles. For instance, a Titan with a scythe - like weapon similar to Ruby's Crescent Rose, and it could have a special ability based on her speed and agility.
One idea could be that Saitama from OPM ends up in the RWBY world and participates in a Grimm - slaying tournament. His overwhelming strength would shock everyone, and he might just be in it for the free food at the event.
For 'rwby multi crossover fanfiction', a crossover with 'Fullmetal Alchemist' would be interesting. RWBY characters could interact with the alchemists and learn about alchemy. They might even find ways to combine their Semblances with alchemy. Also, a crossover with 'My Hero Academia' would be exciting. The RWBY heroes could train with the students at U.A. High and exchange combat techniques and strategies.