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 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.
Definitely Superman and Master Chief. Superman has all his superpowers like super strength, flight, and heat vision, while Master Chief is a highly trained super - soldier. They could have an interesting dynamic, with Superman respecting Chief's combat skills and Chief being in awe of Superman's powers. They could work together to take on powerful enemies, like Darkseid if he somehow got involved in the Halo - Justice League crossover.
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.
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.
One way it could work is by having a threat that requires both the Hulk's strength and the Justice League's diverse powers. For instance, an alien invasion that has technology to nullify the League's powers partially. The Hulk's physical strength would be unaffected, so he could be the key to victory.
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.
One great 'Justice League Crossover Fanfic' is 'Justice League/Teen Titans: Worlds Collide'. It combines the two iconic teams in an exciting adventure. The story has great character interactions and action-packed scenes.
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.