In the context of Spider - Man: Web of Shadows' evil ending fanfiction, one interesting aspect could be the character development. If Spider - Man goes down the evil path, how do the other characters react? For example, Mary Jane might be horrified and try to bring him back to the good side. Also, villains might either team up with this new, evil Spider - Man or see him as a threat to their own plans.
One idea could be exploring what would happen if Spider - Man had to team up with his villains to fight a greater threat from another dimension. For example, Venom and Green Goblin might be forced to work with him.
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'.