One idea could be to combine the unique weaponry of RWBY with the grimdark setting of Warhammer. For example, Ruby's scythe could be enchanted with some sort of Warhammer - style magic or runes. Another idea is to have the characters from RWBY face off against Warhammer - style daemons or chaos warriors. This would really test their skills and teamwork. Also, the concept of semblances could be related to the powers in the Warhammer universe in some way, like a character's semblance being a form of psychic power in the context of Warhammer.
One idea could be a crossover between Warhammer and Lord of the Rings. Imagine the Space Marines facing off against the Orcs of Mordor. Another could be a Warhammer and Star Wars crossover, like the Imperium of Man encountering the Galactic Empire. Or perhaps a crossover with the Witcher, where Witchers team up with Warhammer's warriors to fight common foes.
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 to have the Lizardmen cross over with a modern-day military unit. The contrast between the primitive - yet highly advanced in their own way - Lizardmen and modern military technology would create interesting scenarios. For example, how would the Lizardmen react to firearms? Another could be a crossover with a fantasy world like Middle - earth. The Lizardmen's unique culture and magic could interact in fascinating ways with the races and magic of Middle - earth.
Another plot could be a crossover where the Skaven meet a group of powerful mages from another universe. The Skaven, being cunning, would initially try to deceive the mages and use them for their own gain. But as the story progresses, they find out that the mages are not so easily fooled and a battle of wits and magic ensues.
One idea could be a crossover with the Star Trek universe. The Tyranids could be seen as an extremely hostile alien force that the Federation has to deal with. Their bio - ships and swarming tactics would be a huge contrast to the more technological - based Starfleet.
One idea could be a story where a character from one Total War Warhammer faction gets transported to another faction's territory and has to adapt to survive. For example, a High Elf finding themselves in the midst of a Greenskin horde. They would have to use their wits and combat skills in a very different environment.
A cool plot for Warhammer Fantasy Crossover Fanfiction could be a crossover with the Final Fantasy series. Picture a Warhammer elf meeting a Final Fantasy summoner. They could exchange knowledge about different magical creatures. Also, a crossover with the Naruto universe would be interesting. A Warhammer Chaos warrior might find himself in the Hidden Leaf Village and cause chaos (literally) among the ninja. Or, a crossover with the Witcher world. Geralt could team up with a Warhammer Witch Hunter to hunt down monsters that are a threat to both their worlds.
Another interesting concept is a crossover during a time of great chaos. For example, the warp rifts in Warhammer open up near a key Star Wars system. This causes all kinds of chaos as the daemons from Warhammer pour into the Star Wars universe, and the various factions in Star Wars have to figure out how to deal with these otherworldly threats. It could also explore how the Jedi might try to use the Force to combat the daemonic influence.
One idea could be that Naruto characters are transported to the Warhammer universe. For example, Naruto himself could be mistaken for a powerful chaos warrior due to his chakra. He might then have to prove that he's not a threat while also trying to find a way back home. Another plot could involve the shinobi clans of Naruto being compared to the different factions in Warhammer. The Uchiha with their Sharingan could be seen as having a similar level of mystery and power as some of the Warhammer sorcerers.
One idea could be that the Soul Reapers from Bleach are transported to the Warhammer universe. They might initially be seen as strange entities, and their spiritual powers could be both a mystery and a threat. Maybe they get involved in the battles between different factions in Warhammer, using their zanpakuto and kido to fight against the chaos forces.