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.
Power struggle is a common theme. The Prototype characters often have different and sometimes more brutal powers compared to the Justice League. So, there's always this tension of who is more powerful and how they'll use their powers in a crossover.
Definitely Alex Mercer. His powers are so unique compared to the Justice League members. He can shapeshift into anyone, has superhuman strength, and can regenerate from almost any injury. This would create a lot of interesting interactions with the League. For example, he could transform into Superman to test the limits of his own power against the real Superman, which would lead to a really cool showdown.
One popular 'prototype justice league crossover fanfiction' is 'Prototype Meets the Justice League: A New Alliance'. It combines the dark and gritty world of Prototype with the heroic and diverse Justice League. The story often involves Alex Mercer interacting with Superman and the others, creating some really interesting power - play scenarios.
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.
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.
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.
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.
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.