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.
A great plot idea for a Spiderman Civil War 2 fanfic could be the introduction of a new villain who takes advantage of the chaos of the civil war. This villain could manipulate both sides of the conflict, causing more strife between the superheroes. Spiderman then has to figure out who this new threat is and how to stop them while also dealing with the civil war situation.
Another plot could revolve around the moral dilemmas. Spiderman is often faced with tough choices. In a fanfic for Civil War 2, he could struggle with whether to follow the laws set by the superhero registration side or stay true to his own sense of justice that might oppose some of those laws. This could lead to internal conflict and interesting character development as he tries to balance his beliefs and the expectations of the superhero community.