Well, obviously Spider - Man himself, Peter Parker. He's the main character, and all the stories revolve around him. But other popular characters are his friends like Harry Osborn. Harry has a complex relationship with Peter, and their friendship can be explored in many ways, especially considering Harry's family issues with his father being the Green Goblin. Another popular character is Mary Jane. She can be a love interest, a source of support, or even get involved in some of Spider - Man's adventures.
Another plot idea is a time - travel story. Spider - Man gets sent back in time to when his uncle was still alive. He has to make sure that his actions in the past don't accidentally change the future too much while also trying to learn some valuable lessons from his younger self and his uncle. This could lead to some really emotional moments and also some interesting battles as he might face off against villains that were active during that time in a different way than he did in his own time.
Green Goblin is a must - include character. His chaotic and unpredictable nature can add a lot of action and danger to the fanfiction. Then there's Gwen Stacy. Her relationship with Peter can be explored in different ways, whether it's a romantic angle or just a friendship. Flash Thompson can also be included. He can be used to show the school - life aspect of Peter's life and the bullying he endures, which in turn can fuel Spider - Man's determination to protect the weak.
Maybe a plot where Spider - Man has to face an alien invasion in the context of the 2017 cartoon. The aliens could have unique powers that his usual foes don't, forcing him to adapt his fighting style. He might also have to team up with some of his rogues' gallery who are also threatened by the invasion. This would create an interesting dynamic as they have to put aside their differences to save the city and the world.
It all starts when Peter Parker gets bitten by a radioactive spider. This gives him amazing powers like super strength, the ability to climb walls, and a 'spider sense'.
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.