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.
Well, in this fanfic, the Hulk could bring a new dynamic to the Justice League. His lack of restraint might force the League to adapt their strategies. For example, Batman would have to figure out how to control Hulk's rage. And Wonder Woman might find herself in a position where she has to protect Hulk from his own self - destructive tendencies while also using his power to their advantage. It could be a story full of conflict and cooperation.
In a Justice League Avengers crossover fanfic, there could be epic battles. For example, Superman and Thor might have a showdown to test their super strength. They could exchange powerful blows while the other heroes watch in awe.
One idea could be that Alex Mercer from Prototype finds himself in the Justice League's universe. He could initially be seen as a threat by the League. But then, they realize his power could be used to fight a common enemy, like Darkseid. Mercer's shape - shifting and regenerative abilities would add an interesting dynamic to the team's combat strategies.
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.
One idea could be that the Covenant from Halo discovers Earth while the Justice League is busy dealing with a local threat. The League has to quickly adapt to this new alien threat that has far more advanced technology than most of their foes. They might have to team up with Master Chief, who is also on a mission to stop the Covenant's invasion plans.
Teamwork is another big theme. In these crossovers, the Justice League has to learn to cooperate with new characters. They often start off with some misunderstandings, but then they come together to face a greater threat. It's all about how different heroes can combine their skills for a common good. In many fanfics, there's also a theme of exploring new worlds. When the Justice League crosses over, they get to see different universes, cultures, and ways of doing things in the other team's world.
Maybe the plot could involve a device created by Lex Luthor that accidentally merges the two universes. Spider - Man then teams up with the Justice League to find a way to separate the universes again. During this time, he has to deal with the different personalities and powers of the League members. He might form a special bond with Batman, as they both rely on their wits and gadgets.
Well, it could be amazing. Picture Spider - Man's acrobatic fighting style combined with the superpowers of the Justice League members. Maybe he teams up with Superman to save a city from a huge threat. Their different personalities would create interesting interactions, like Spider - Man's quippy nature against Superman's more serious and noble demeanor.
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.