Gwen Stacy is an obvious choice. In different universes, she has different relationships with Spider - Man. Another could be Mary Jane. She has always been an important part of Spider - Man's life. Black Cat might also be in the harem as she has a complex and flirtatious relationship with Spider - Man in the comics.
One interesting concept could be the different relationships that Spider - Man forms with various characters in the harem. For example, his interactions with different versions of Gwen Stacy or Mary Jane could be explored in unique ways. Maybe there are stories where they all team up to fight villains together while also dealing with their complex feelings for Spider - Man.
One possible fanfiction could be about Venom finding its way into the Spider - Verse. Maybe it starts when a symbiote experiment in an alternate dimension goes wrong and Venom is accidentally sent into the Spider - Verse. He could encounter different versions of Spider - Man and create chaos as he tries to find a way back to his own dimension. He might initially be seen as a threat by the Spider - People, but as the story progresses, they could realize that he's just lost and confused.
One idea could be a story where Miles Morales has to team up with an alternate version of himself from a dimension where he never became Spider - Man. They have to overcome their differences and learn from each other to stop a new villain that is threatening all the spider - verse.
In Spider - Man Into the Spider - Verse story, it begins with Miles Morales. He's a young boy with his own set of insecurities and dreams. When he gains his spider - like abilities, he has to learn to control them quickly. The story takes a wild turn when the multiverse is involved. There are various versions of Spider - Man, each with their own unique styles and personalities. For example, there's a Spider - Man who is a noir - style detective. Kingpin's actions disrupt the balance of the multiverse. Miles, along with the other Spider - People, has to face many challenges, not only in fighting Kingpin but also in understanding and accepting his new role as a Spider - Man. It's a story full of action, humor, and heart - warming moments as Miles grows into his new identity.
It changes the original story by introducing the concept of the multiverse. In the normal Spider - Man story, he's mainly dealing with threats in his own city. But in the fanfiction within the MCU, he has to face threats from different universes. This makes his challenges much bigger and more complex.
One idea could be exploring the backstories of the different Spider - People in more detail. For example, how did Peni Parker really come to terms with her role as Spider - Woman in her own dimension. Another could be a crossover within the Spider - Verse, like what if Miles Morales and Gwen Stacy teamed up to face a new villain that was causing chaos across multiple dimensions.
You can't really 'watch' fanfiction in the traditional sense like you do a movie. Fanfiction is written text. However, you can find Spider - Man: Into the Spider - Verse fanfiction on various fanfiction websites such as Archive of Our Own (AO3) or FanFiction.net. Just search for the relevant tags related to the movie on these platforms.