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.
For a Watch into the Spider - Verse fanfiction, you could write about the origin story of a new Spider - Person in one of the many dimensions. This character could be an ordinary person who gets bitten by a radioactive spider but has a unique power that sets them apart from the other Spider - People. They then cross paths with Miles and the gang and have to prove themselves worthy of joining the team. Another idea is to explore a world where the spider - powers are not just physical abilities but also grant the wielder magical powers, and how this changes the dynamic of the Spider - Verse.
The plot might involve the Avengers seeing the multiverse aspect in Into the Spider - Verse and realizing the vastness of the universe. They could then start to look into the possibilities of other multiverses and if there are other versions of themselves. This could lead to a whole new adventure where they try to contact or visit these other universes, all inspired by what they saw in the Spider - Verse.
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.
Thor would be a bit more straightforward. He'd probably see the Spider - People as brave warriors. He might be a bit confused at first about the multiverse concept but would quickly accept it. He'd be like 'These small heroes have great courage, much like the warriors of Asgard.' And he might offer to train them if they ever met. He'd also be entertained by the humor in the Spider - Verse, like the interactions between Peter B. Parker and Miles.