Perhaps in this fanfic's Civil War, Cable and Deadpool are leaders of two different mutant factions. Cable's group wants to form an alliance with humans for mutual protection, while Deadpool's crew is more isolationist. Their conflict could center around a particular piece of technology or a young mutant with special powers that both sides think they can use to tip the balance of power. Deadpool could be constantly cracking jokes during the serious battles, adding a touch of his unique humor to the intense situation.
One idea could be that Cable and Deadpool find themselves on opposite sides due to a misunderstanding about a new mutant - registration law. Cable believes in strict control to protect mutants in the long run, while Deadpool just wants chaos and freedom. Maybe Deadpool starts a prank war against Cable's side that accidentally escalates the situation.
A possible plot could involve an external force manipulating Cable and Deadpool into thinking the other is a threat. For example, a powerful villain uses mind - control technology. As they fight each other, they start to realize that something is off. This could lead to them teaming up to find the real enemy. In the process, they have to deal with their own ideological differences as well as the damage they've already caused to their friendship.
Mr. Sinister could be a great antagonist. He's always interested in mutant genetics and power. He could be the one secretly pulling the strings, making Cable and Deadpool believe they need to fight over the future of mutantkind. He could plant false evidence or use his mind - control abilities on some of the supporting characters to fuel the conflict.
One idea could be exploring new alliances. For example, Spiderman might team up with some unexpected heroes during the civil war. Maybe he forms a bond with a young, new hero who is caught in the middle of the conflict.
Maybe in the fanfic, Hulk has a different view on the registration act. He could see it as a form of control, like how he was always being hunted. So he sides with the anti - registration side, but not without some internal struggle. After all, he doesn't fully trust any of the heroes at first. He might end up teaching them a lesson about freedom and acceptance.
You might consider writing a civil war fanfic about a group of spies. They could be infiltrating enemy camps and trying to gather intelligence. Their actions could change the course of certain battles. For example, they might discover a secret plan for a major offensive and find a way to thwart it.
One idea could be exploring the perspectives of the minor characters during the conflict. For example, how an ordinary citizen in Sokovia feels about the heroes' actions. Another could be a what - if scenario where Steve Rogers and Tony Stark managed to resolve their differences before the big fight. Maybe a story about a new superhero who emerges during the civil war and tries to mediate between the two factions.
One idea could be exploring how the fractured Avengers try to regroup on their own. For example, Captain America could be leading a small, underground team that focuses on helping those affected by the Sokovia Accords in a more discreet way. Meanwhile, Iron Man might be working with the government to create a new, more controlled version of the Avengers.
One idea could be that the Justice League members have different views on how to handle a new superpowered threat. Some might want to use force immediately while others prefer a more diplomatic approach. This could lead to a split in the league, with each side believing they are in the right.
One idea could be that a new hero emerges during the Civil War and tries to find a middle ground between the two factions. They might have unique powers that allow them to communicate with both sides on a deeper level, and they end up uniting some of the heroes against a common, unexpected threat that was using the Civil War as a distraction.
Perhaps Spider - Man could create his own team of young superheroes who are inspired by the ideals of both sides. His team could then try to show the older heroes that there is a middle ground. They could start by doing small acts of heroism that don't take sides but help the general public. This would attract the attention of both Team Cap and Team Iron Man and make them question their own stances in the Civil War.
Another plot idea is that Peter's identity as Spider - Man could be at risk in a different way during the events of the Civil War in the fanfic. Maybe someone on the opposing side discovers his identity and tries to use it against him, not to expose him publicly, but to manipulate his actions during the war.