Peter could get a dog. A loyal dog would be a great companion for him, following him around during his daily life and even being a sort of sidekick. It could sense danger and bark to alert Peter when something was wrong.
One idea could be that Peter Parker gets a prosthetic after a particularly dangerous battle with a villain. Maybe his leg is severely injured and he has to adapt to the new prosthetic while still maintaining his superhero duties. He could struggle at first with the balance and movement, but with his determination, he eventually masters it and even modifies the prosthetic to have some special features like enhanced jumping ability.
It could be a vengeful criminal that Spider - Man has put away in the past. This criminal has spent years in prison plotting his revenge and finally gets the chance when Peter is vulnerable. Maybe he manages to ambush Peter in an alleyway while he's on his way home from school or work.
In 'peter is hit by a train fanfiction', it could be a very dark and dramatic story. The train hitting Peter could be the start of a journey into the afterlife. He might meet strange entities or have to face his own inner demons. His physical body could be in a critical state in the real world while his spirit is on this otherworldly adventure.
In some fanfictions, Peter Parker might use his super strength to hold onto the tracks and gradually slow the train down with sheer force. He could also shoot his webs at various structures near the track to create a makeshift braking system.
The 'Peter Parker Gets Stitches' fanfiction might be centered around the idea of vulnerability. Peter is often seen as a strong superhero, but getting stitches shows his human side. It could be about how he copes with this moment of weakness. There could be a lot of internal monologue as he sits in the hospital or wherever he gets the stitches, thinking about his responsibilities and how this injury might impact his future as Spider - Man.
His personality might become more in touch with his emotions. As a girl, Peter could be more empathetic and sensitive. He might also be more cautious in some situations, as society often treats girls differently. For example, he might not be as quick to jump into a fight without thinking about the consequences more carefully.
Perhaps his best friend. This friend could be the one who was with him right before the accident and blames himself for not stopping Peter from getting too close to the tracks. He would then be a key character in the story, trying to make amends in some way while also being there for Peter's family.
Peter Parker is the main character for sure. In addition to Aunt May and MJ, there could be other characters from the Spider - Man universe. For example, Doctor Strange could make an appearance if the injury has some sort of magical or other - worldly aspect to it. Or perhaps Iron Man's influence could be felt if the story is set in a universe where they have interacted. And let's not forget the Daily Bugle crew, like J. Jonah Jameson, who might be looking for a story about Spider - Man's injury.
In such a fanfiction, Peter might lose his usual control. He could accidentally reveal his identity while drunk. For example, he might start talking about his Spider - Man adventures without realizing it.