Maybe Peter discovers a new and powerful criminal organization that has infiltrated the city at a very deep level. He realizes that if he involves others, it could put their lives in danger, so he decides to take on this threat alone. This could lead to a series of dangerous encounters where he has to use all his wits and powers.
Well, in this kind of fanfiction, Peter is likely portrayed as a lone hero. He might be facing all sorts of challenges on his own while fighting crime. Maybe he doesn't trust the authorities or his usual allies for some reason, so he decides to take matters into his own hands. It could be a really interesting take on the character, showing his independent and determined side.
A major theme is the sense of responsibility that Peter feels. He has this power and he knows he has to use it to protect the city. This often leads to him overextending himself. Another common theme is the mystery surrounding the criminals he faces. Some fanfictions have complex criminal masterminds that Peter has to outwit. And then there's the theme of the city itself as a character. The city is both the place he's protecting and also the source of many of the problems he has to deal with, like poverty - stricken areas that breed crime.
Sure. One great piece of 'peter fights crime fanfiction' is 'Peter's Night Patrol'. In it, Peter uses his wits and unique abilities to take on a gang that's been terrorizing the neighborhood. Another one could be 'Peter vs the Crime Syndicate' where he has to face a powerful group of criminals.
One possible scenario in such a fanfiction could be a school - related incident. Flash taunts Peter in front of everyone, and Peter, tired of it, decides to fight back in a more subtle way. Maybe he uses his super speed and reflexes to dodge Flash's attacks and make Flash look foolish without using full - on Spider - Man powers.
In some 'peter is alone fanfiction', he could be alone in a strange new place. For instance, he's transported to a different dimension where he doesn't know anyone. He has to adapt, survive, and maybe find a way back home all by himself. This kind of plot can explore his character growth, his resourcefulness, and how he deals with the isolation in a completely unfamiliar environment.
Perhaps Peter is alone in a forest, away from the city. He's there to find some peace and quiet, to get away from the chaos of New York. While he's alone, he reflects on his powers and how they've both blessed and cursed him. He might even start to train in new ways, using the seclusion of the forest to his advantage.
Maybe an ancient evil ninja who was sealed away long ago but has now been released. This antagonist could have powers that nullify any communication or summoning jutsu, forcing Naruto to fight alone. He would be extremely powerful and know many forgotten and deadly jutsu.
In a 'naruto fights alone fanfiction', it could be that Naruto is on a mission in a foreign land where he has no support. The local ninjas are all against him for some reason. He has to use his Nine - Tails chakra in a more discreet way as he can't afford to go all - out and draw too much attention. He might befriend some local animals or spirits who help him in a more indirect way, like giving him information about the enemies' weaknesses.
In a 'Harry Fights Alone' fanfiction, the plot could involve Harry being betrayed by his friends. Maybe they are put under a spell that makes them turn against him. Harry then has to face the dark forces all by himself. He might have to rely on his own wits and the few spells he has mastered alone. For example, he could be in a situation where he has to battle Voldemort's minions without Ron and Hermione to back him up, and he discovers new powers within himself during this struggle.