A plotline could be that Peter has a falling out with his team or friends over a moral or ethical issue. He then decides to go solo in his crime - fighting efforts. As he fights alone, he starts to question his own beliefs and values, and through his solo adventures, he either reaffirms or changes them while still trying to keep the city safe from various villains.
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.
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.
One possible plotline could be a fight over a love interest. Maybe they both like the same vampire or human and it leads to a physical or verbal fight between Bella and Tanya.
One possible plotline could be Peter having to use his wits more than his powers. Maybe the Sinister Six has some new technology that nullifies his usual spider - abilities, so he has to rely on his intelligence and the environment around him to fight. For example, using the city's infrastructure to his advantage like swinging from unexpected places or using hidden passages.
One possible plotline could be Peter struggling to communicate his superhero identity and his daily life challenges to his friends and family without using his voice. Maybe he has to develop a unique sign language or other non - verbal communication methods.
One possible plotline could be that Luke has been corrupted by some dark force, and Percy has to fight him to save their world. Percy might be reluctant at first because they were once friends, but the greater good forces him into action.
A possible plotline could be Finn getting lost in a magical forest all by himself. He has to find his way out while facing strange creatures and magical obstacles.
Maybe in the fanfiction, Tony discovers Peter's potential not just as Spider - Man but also in other areas like science. So he starts mentoring Peter more intensively, leading to a closer father - son type of relationship. They could work on new inventions together, with Peter learning a great deal from Tony's experience.
A possible plotline could be that Peter's identity as Spider - Man is at risk of being exposed, and Jessica Jones helps him cover it up. She uses her skills as a private investigator to mislead those who are getting too close to the truth.