Well, a great storyline might be the symbiote causing Spider - Man to have moral dilemmas. Since the symbiote can be a bit dark, it could push Spider - Man to cross lines he normally wouldn't. For example, using excessive force against criminals. Also, there could be a story where the symbiote's origin is explored more deeply in the fanfiction. Maybe it has a connection to an ancient evil that Spider - Man has to uncover. Another idea is that the symbiote starts to affect Spider - Man's perception of reality, making him see enemies where there are none at first.
One interesting plotline could be the symbiote influencing Spider - Man's moral compass in ways he didn't expect. For example, it might make him more aggressive in fighting crime, but also more willing to cross ethical lines. Another could be the symbiote's memories from its home planet affecting Spider - Man's understanding of the universe. It might show him visions of otherworldly battles or technologies that he then tries to incorporate into his own crime - fighting methods. A third plotline could involve the symbiote bonding with other characters in the Spider - Man universe, like Mary Jane or Ned, and how that changes their relationships with Spider - Man and their own roles in the story.
The symbiote often makes Spider - Man more aggressive. He might be quicker to anger and less patient. For example, he could snap at his boss J. Jonah Jameson more often.
Well, there could be a fanfiction where the Sinister Six creates a clone of Spider - Man. This clone is programmed to do their bidding and fight against the real Spider - Man. The real Spider - Man then has to prove his authenticity to the people of New York while also trying to stop the clone and the Sinister Six. Another idea is that the Sinister Six uses mind - control technology on some of Spider - Man's friends and allies, forcing Spider - Man to choose between fighting his friends or finding a way to break the mind - control.
A plot idea could involve the symbiote suit being able to communicate with other symbiotes from outer space. It could be luring them to Earth, thinking they can all live in harmony, but they actually want to take over the planet. Spider - Man has to figure out a way to stop this while also dealing with the fact that the suit is becoming more and more unruly as it gets closer to its 'friends'.
Spider - Man could develop a new sense of confidence, but also arrogance. The symbiote suit makes him stronger and more powerful, so he might start to think he can handle any situation alone. This could lead to him pushing away his friends and allies, like Mary Jane or Harry Osborn, until he realizes the error of his ways.
One interesting storyline could be about how Spider - Man manages his relationships with different characters in the harem while still fighting crime. Maybe he has to balance his time between going on dates with his love interests and saving the city from villains.
One unique storyline could be a different origin for one of Spider - Man's villains within the context of this fanfiction. Maybe the Green Goblin has a new backstory that ties into a lemon grove, which gives him his powers in a strange and unexpected way.
One great storyline could be where the symbiote enhances Spider - Man's relationship with Black Cat. Maybe the symbiote gives him an edge in protecting her during a new wave of villain attacks in the city. For example, it could make him faster and stronger when fighting off thugs who are after Black Cat for some reason.
One popular storyline is Venom teaming up with other villains. This creates a bigger threat for Spider - Man. Another is Venom trying to take over the city. His symbiotic powers give him an edge in such a plot.
One good storyline could be Black Cat and Spider - Man teaming up to take down a new super - villain in the city. Maybe a villain who has the ability to steal powers, and they have to figure out how to stop him while also dealing with their own complicated relationship.