It could be his mentor. In the fanfic, the mentor might think that Spider Man is becoming too powerful and is a threat to the balance of power. So, he could betray Spider Man by leaking his secret identity or setting him up to be captured by the authorities.
Even one of Spider - Man's fellow superhero friends might betray him. Maybe they get jealous of Spider - Man's popularity or feel that he is getting all the credit for their team's successes. This internal betrayal would add a lot of drama and conflict to the fanfiction story.
He would be very hurt. He might isolate himself for a while, doubting his own judgment in trusting his friends. Spider - Man might also start to question his identity as a hero. After all, if his friends can turn on him, what does that say about his relationships and his role in the world?
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.
One possible plotline could be that Spider Man is betrayed by his closest friends in the fanfic. Maybe they believe false rumors about him being a villain or working for the enemy. This could lead to a lot of action-packed scenes where Spider Man has to prove his innocence while also fighting against the bad guys.
Iron Man is a great character to pair with Spider - Man. Tony Stark can act as a mentor to Peter Parker, providing him with new and advanced tech for his suit. They also have a fun dynamic, with Stark's sarcasm and Spider - Man's youthful exuberance.
An anti - hero or a morally grey character might be the antagonist. This character could have a different view on relationships and superhero life. He convinces Spider Man that the harem is holding him back from achieving greater things. And Spider Man, being influenced, betrays the harem.
Venom is quite popular in Spider - Man fanfiction. His complex relationship with Spider - Man can be explored in many ways. He can be a villain, an anti - hero, or even an ally in some really creative fanfictions. Another popular one is Gwen Stacy. Fans love to write about her and Peter's relationship, especially in alternate timelines.
His best friend could be the one. Since they are close, they might notice some strange behavior or coincidences that lead them to suspect Peter is Spider - Man. For instance, Peter always disappearing at the same time Spider - Man shows up.
One possible villain could be Doctor Octopus. He might be back with some new and improved mechanical arms, and he wants to prove that he's the superior scientist and villain by defeating both Spider Man and Spider Girl.
One example could be a story where Spider Man is betrayed by his friends in the Avengers. Maybe they suspect him of a crime he didn't commit due to some false evidence planted by a villain. Another could be a scenario where his love interest, like Mary Jane, betrays him by revealing his secret identity to the media out of jealousy or being manipulated.