Peter is obviously one of the main characters since the story is centered around him turning to dust. His friends could also be main characters as they are likely to be involved in the events that follow his transformation.
In some 'peter turns to dust fanfiction', there might be a story where the cause of him turning to dust is explored in a unique way. For example, it could be due to a new kind of magic or an ancient curse that no one knew about before. His friends then embark on a journey to discover the origin of this curse and find a cure.
Well, in a fanfiction where Peter Parker turns into a kid, perhaps he could end up at his old school. He'd be faced with bullies again, but this time he has the knowledge and experience of being Spider - Man, even if he's in a kid's body. This could lead to some interesting scenarios where he uses his smarts to outwit the bullies without using his powers directly. Or, he could befriend his younger self's friends in a whole new way, seeing them from a different perspective.
MJ (Mary Jane) is often a common character. She would have a very different dynamic with Peter as a girl. Also, Aunt May is usually there. Her relationship with Peter would likely change in interesting ways, as she might have different concerns and advice for a female Peter. Another common character could be Ned. His friendship with Peter would be different, and he might be one of the first to notice the changes in Peter's behavior due to the gender swap.
Well, the consequences could be far - reaching. In the superhero world, his absence as the full - fledged Spider - Man could lead to a power vacuum. Villains might become bolder, thinking Spider - Man is gone. On a more personal level, Peter as a kid might struggle with his identity. He's used to being a responsible superhero, but now he's a child again. He might have to relearn how to control his powers as they could be affected by his new physical state. And his relationships with love interests would also be affected. They might see him as a cute kid rather than the man they were attracted to, which could create some really interesting and complex storylines.
In some 'peter doesn't turn to dust fanfiction', the plot might revolve around a character coming to Peter's rescue at the last moment. This could be a character with unique powers or knowledge. They could use their abilities to reverse the process that would make Peter turn to dust. Or perhaps there's a magical or technological device that is introduced in the story that stops Peter from turning to dust. The fanfiction could then explore how Peter and the other characters deal with this new situation and what new challenges or opportunities it presents.
One idea could be that Peter, now as a female, has to navigate the daily challenges of high school in a completely different way. Maybe she discovers new powers related to femininity that the male Peter didn't have. For example, she could have a heightened sense of intuition that helps her in battles or in understanding her friends' emotions better.
One possible plot could be that Peter Parker's transformation into a spider is a permanent one. He has to deal with not just his regular superhero duties but also the new challenges of being part - spider all the time. Maybe he has to learn new ways to communicate with other spiders to gain some special knowledge or power.
One place to look for such fanfiction could be Archive of Our Own (AO3). There are often unique takes on characters' fates in fan-created stories there.
Well, perhaps in the fanfiction, Peter's memories as an adult are still somewhat intact in his baby form. So he tries to communicate with his loved ones in his own baby - like ways, like making specific noises when danger is near or trying to write in a very clumsy way. This could lead to some really cute and interesting interactions.