A great crossover aspect could be the Precure's magic interacting with the Warhammer magic or technology. The Precure's spells could have unexpected effects when used in the Warhammer setting. Maybe their purification magic could be used to cleanse some of the corrupted areas or creatures. And they could also learn from the Warhammer characters, like how to better defend against certain types of attacks in this dangerous universe.
In a Halo x Warhammer fanfiction, there could be a scenario where the Flood from Halo is unleashed in the Warhammer universe. This would be a huge disaster as the Flood has the ability to infect and assimilate almost any living being. The Imperium of Man in Warhammer would have to come up with new strategies to deal with this alien threat. Maybe they would try to use their psykers to combat the Flood's hive - mind. And the Halo UNSC forces might also get involved, bringing their own technology and tactics to the fight.
One interesting crossover could be the characters from Zootopia in the Warhammer universe. For example, Judy Hopps could be a scout in the Imperial Guard. Her natural agility and detective skills would make her a great asset. Another could be Nick Wilde using his wily ways to navigate the political intrigue of the Warhammer factions. He might be able to get information or make deals that others couldn't.
In Halo x Warhammer 40k fanfiction, there might be crossovers where the Forerunners' technology from Halo is discovered by the Imperium of Man in Warhammer 40k. This could lead to a power struggle as different factions within the Imperium vie for control of this new and powerful technology. Also, the Flood from Halo could pose a unique threat to the Warhammer 40k universe, and seeing how the various races in that universe would deal with it would be really fascinating.
A really cool concept could be the Warhammer magic items being introduced to the Fairy Tail world. Imagine if Gray got his hands on a Warhammer frost weapon. It could enhance his ice - make magic in ways he never thought possible. And the different magical lore from Warhammer could inspire new spells among the Fairy Tail mages.
Well, think about it. The Precure are all about friendship, love, and protecting the world with a smile. In Warhammer, though, survival is often the main goal. But if you put them together, the Precure's personalities could start to break down the walls that the Warhammer characters have built up. For example, a Precure might befriend a Warhammer character who has always been alone, and show them the value of having someone to rely on. And the Warhammer character's more practical and battle - hardened nature could also teach the Precure about the harsher side of reality, making their own personalities more well - rounded.
A common crossover is the appearance of Starfleet officers in the Warhammer 40k universe. They might be seen as strange and otherworldly beings. Their peaceful exploration ethos would be a sharp contrast to the constant warfare in 40k. For example, they could try to broker peace between warring factions in 40k, which would be a huge challenge given the nature of that universe.
One interesting crossover could be the use of Warhammer's magic items in the Harry Potter universe. For example, a Horcrux could be compared to a Chaos-tainted artifact in Warhammer. Another could be the appearance of Warhammer races in Hogwarts. Maybe some Orcs could accidentally end up in the Forbidden Forest and cause chaos there.
One interesting crossover could be the use of Percy's water - based powers in the Warhammer Total War setting. Imagine Percy being able to manipulate the water bodies in the game's maps to his advantage, like creating floods to disrupt enemy formations. Another aspect could be the integration of the Greek mythology elements from Percy Jackson into the Warhammer universe. For example, the gods in Percy Jackson could have counterparts or interact with the deities in Warhammer in some way.
When Naruto interacts with the different races in Warhammer, like the Space Marines or the Orks. The cultural clashes and the way he forms alliances or rivalries are really engaging. For instance, his initial confusion about the Space Marines' strict code of conduct and then gradually learning to respect and work with them is a great aspect of some fanfics.
One idea could be creating an OC who has the power of music in the Precure world. They could use melodies to heal and fight against the villains. Another is an OC from a different dimension who stumbles into the Precure universe and has to learn their ways to help. And also, an OC that is a hybrid of a magical creature and a human in the Precure world, bringing unique powers and a fresh perspective to the story.