A more character - based story could involve Iron Man and an Imperial Tech - Priest. They could start off as rivals, each thinking their technology is superior. But then, when faced with a common enemy like a horde of Tyranids, they have to combine their knowledge. Iron Man's high - tech suits and the Tech - Priest's understanding of the arcane technology of Warhammer 40k could create some really cool gadgets and strategies to fight off the alien threat.
Another idea is a crossover where Thanos tries to get his hands on some of the powerful artifacts from Warhammer 40k. Maybe he wants the Emperor's sword or one of the powerful relics from the Adeptus Mechanicus. This would lead to a conflict not only between him and the Warhammer 40k forces but also potentially with the Guardians of the Galaxy who are also after these powerful items.
One interesting idea could be a story where the Space Marines from Warhammer 40k team up with the Avengers. Picture the Ultramarines and Captain America leading a combined force against a chaos incursion in New York. The Ultramarines' discipline and advanced weaponry combined with the Avengers' superpowers would make for an epic battle.
One idea could be that the Infinity Stones are somehow connected to the Warp in the Warhammer universe. Maybe Thanos discovers a way to tap into the power of the Warp through the Stones, which brings the heroes of Marvel into conflict with the daemons and factions 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 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.
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 the X - Men and the Avengers. Imagine Wolverine teaming up with Captain America in a battle against a common enemy like Ultron. Another could be a Spider - Man and Doctor Strange crossover, where Spider - Man's quick thinking and agility combine with Doctor Strange's magic to solve a multiverse - level threat.
Another interesting concept is the Flood from Halo infecting some of the Marvel characters. This would force the heroes to find a way to stop the spread of the Flood, which is a very different kind of threat compared to what they usually face. They might have to work with UNSC scientists and soldiers to develop a cure or a way to contain the Flood.
Thor and a Dwarf Lord would be really interesting. Thor with his god - like powers and his hammer Mjolnir could team up with a Dwarf Lord, who is known for his skill in forging and combat. They could exchange stories about their great battles and also work together to create some powerful weapon, perhaps using Asgardian and Dwarven forging techniques.
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.
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.
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.