Another possibility is a more comical crossover. The Flash tries to race a Warthog from Halo and fails miserably because he doesn't understand the vehicle's controls. Or Batman trying to figure out the technology of Halo's weapons and getting all confused. This kind of fanfic would be full of humorous moments and cultural clashes between the two universes.
Character development is key. If you're writing a fanfic about this crossover, you need to make sure the characters from both universes stay true to their original personalities while also evolving in the new situation. For instance, Wonder Woman should still be strong - willed and brave when she meets the Arbiter from Halo.
One idea could be that the Young Justice team has to prove themselves to the Justice League during a major alien invasion. The League initially doubts their abilities but as the battle progresses, they see the unique skills and teamwork of the young heroes.
In a Danny Phantom and Justice League crossover fanfic, Danny could team up with Superman. Maybe Danny uses his ghost powers to help Superman when he's facing a powerful ghost - like enemy that the Justice League can't handle with their normal tactics. It would be a really cool combination of the two universes.
Develop their relationships. Maybe a Young Justice hero has a huge crush on a Justice League member, which creates some interesting dynamics. Also, showing how they interact in battle, like a Young Justice member learning new combat techniques from a League veteran.
Well, prototype crossover fanfic is basically when fans take the idea of 'prototype' and mix it up. They might take characters from one 'prototype' - based story and put them in the world of another. For example, if there are different 'prototype' games with their own unique features, a fanfic could crossover the characters from those games. It's a form of fan creativity to explore new stories and interactions that the original creators might not have thought of.
Well, Superman's strength and invulnerability would be a great match against the brute force of some of the larger Warhammer creatures like Ogres or even some of the daemons. His heat vision could also be a useful weapon against hordes of enemies. Batman's strategic mind would be invaluable in dealing with the complex politics and power struggles in the Warhammer universe. He could also use his gadgets to adapt to the different technologies and magics there. Wonder Woman's combat skills and her Lasso of Truth could be used to great effect in dealing with the more devious characters in Warhammer, like the Dark Elves.
One exciting plot twist could be that the Chaos forces from Warhammer somehow corrupt a member of the Justice League. Imagine Superman, under the influence of Chaos, turning against his comrades. It would create a huge internal conflict within the League as they struggle to save him without harming him. Another could be that the technology or magic from Warhammer is introduced to the DC Universe in a way that changes the power dynamics. For example, the Warp magic could give new abilities to some of the villains, forcing the Justice League to adapt quickly.
One idea could be that Voldemort discovers a way to enter the DC universe and teams up with some of the Justice League's villains like Lex Luthor. The Hogwarts students and the remaining members of the Justice League have to join forces to stop them. The wizards can use their magic while the superheroes use their powers.
In fanfics involving Catwoman in the Justice League, she often brings a unique dynamic. Her morally grey nature contrasts with the more traditional heroes. For example, she might be involved in a heist that has unexpected consequences for the Justice League. Her skills as a thief and her acrobatic abilities make her a formidable character in these fictional stories.