One possible plotline could be that Peter Parker and Jill Valentine are brought together by some kind of multiverse event. They then have to team up to fight a common enemy that threatens both their worlds. For example, a new villain who has the ability to spread a virus in Peter's world and summon zombies in Jill's world.
Another plot could be that Jill Valentine is somehow transported to Peter Parker's New York. She gets involved in the superhero drama there, and Peter is initially suspicious of her but then realizes she's a capable fighter. As they work together to keep the city safe, feelings start to develop between them. Their different combat styles - Peter's acrobatic and web - slinging vs Jill's more gun - based and tactical - could also make for some interesting action scenes in the fanfic.
You could also try Archive of Our Own (AO3). It has a great tagging system. Just search for 'Peter Parker' and 'Jill Valentine' in the search bar, and it'll show you all the relevant fanfics. Some of them might be really creative, exploring different scenarios where these two characters from different universes interact.
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.
A plotline could involve Peter having a major falling out with his loved ones. For example, if he feels betrayed by his friends or Aunt May, he could turn evil. He might then use his knowledge of the superhero community to target other heroes and villains alike, causing chaos in the city. Another idea is that an outside force, like an alien symbiote or a powerful magic, could influence his mind and make him evil, leading to a series of events where he tries to reshape the world according to his now - twisted views.
One possible plotline could be that Peter is poisoned by one of his enemies, like the Green Goblin. He then has to struggle to find an antidote while still trying to keep his identity a secret. Maybe he uses his scientific knowledge to analyze the poison and find a cure, all while fighting off the effects of the poison.
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.
One possible plotline could be that Elsa, with her ice powers, somehow ends up in Peter Parker's world. Maybe she's on the run from some threat in her own realm and Peter, being the friendly neighborhood Spider - Man, decides to help her. They could form a bond as they both have secrets to keep - Elsa with her powers and Peter with his identity as Spider - Man.
Maybe it could explore the relationships around her. How does her love interest react? Her friends and family? It could be a story about their support or lack thereof. Also, it could involve her powers being affected by the pregnancy in some way, like having to learn new ways to use them or facing limitations.
One possible plotline could be that Peter Parker gets sick with a mysterious illness that even his enhanced healing can't quickly fix. Dr. Bruce Banner, with his scientific expertise, tries to find a cure. Along the way, they bond over their shared experiences of being different and having to deal with the consequences of their powers.
A possible plotline could involve their combined powers. They might use their abilities in unique ways to protect their family or the community they live in. It could also be about how they balance their public and private lives, especially since they both have extraordinary abilities.
One possible plotline could be Peter Parker using his spider - abilities to rescue people trapped in the collapsing building. He might swing in and out, carrying people to safety one by one. Another could be that Peter has to deal with his secret identity while trying to save the day during the building collapse. For example, he has to find a way to change into his suit without being noticed, or he might be recognized by someone he knows while in the middle of the rescue operation.