One idea could be Carnage taking over a major corporation in the Spectacular Spider - Man universe. Spider - Man then has to figure out how to stop Carnage without causing too much destruction to the company and its employees. Another plot could involve Carnage teaming up with a lesser villain to create chaos, and Spider - Man has to face two foes at once.
One idea could be exploring the relationship between Spider - Man 2099 and other superheroes in the future. Maybe he teams up with a new group of heroes to fight a powerful villain who threatens the entire city.
One idea could be that Female Carnage targets Spider Man's loved ones to draw him out. Spider Man then has to use his wits and powers to protect them without getting caught in Carnage's violent traps.
Well, perhaps in the fanfiction, Spider - Man bonds with Carnage to save an innocent person. Let's say Carnage has taken a hostage, and Spider - Man discovers that he can form a temporary bond to influence Carnage's actions. He then uses this bond not only to save the hostage but also to try and reform Carnage in some way. It could also be that the bond is a result of a magical spell gone wrong in the Marvel universe that links their fates together.
One idea could be exploring a different origin story for Spider - Man in the fanfiction. Maybe he gets his powers from a scientific experiment gone wrong in a completely new way. For example, instead of being bitten by a radioactive spider, he could be exposed to a mutagenic serum that gives him spider - like abilities along with some unique side - effects that he has to deal with.
Another plot could involve the black suit affecting Peter's relationships. Maybe his friends start to notice the change in him, and he has to deal with their concerns. Mary Jane might be worried about the new, edgier Peter. Also, the suit could cause problems with his identity as Spider - Man. What if it starts to take over during his daily life and he has to struggle to keep it in check? This could lead to some really interesting internal and external conflicts.
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'.
One plot idea could be that Liz discovers Peter's identity as Spider - Man in a really unexpected way. Maybe she accidentally stumbles upon his suit while looking for something in his room. Another could be that they team up to solve a mystery in their school that the police can't figure out, using Peter's powers and Liz's intelligence.
One plot idea could be Spider - Man taking a break at the beach and suddenly having to face a new villain who emerges from the ocean depths. Maybe it's a mutated sea creature caused by some underwater experiment gone wrong. Another could be that Peter Parker is at the beach with his friends and he has to balance his normal life and secret identity when a small - scale crime occurs right there on the beach. He has to find a way to stop it without revealing his powers to his friends.
A great idea for a Spectacular Spider - Man fanfic is to introduce a new villain. This villain could be someone with a personal vendetta against Peter Parker, not just Spider - Man. It could be a former classmate who blames Peter for something that happened in the past. You could also write a fanfic about Spider - Man teaming up with other superheroes from the Marvel universe. He could join forces with Daredevil to take on a common enemy in New York. Or perhaps a story where Spider - Man has to deal with the consequences of his actions in a more emotional way, like when he fails to save someone and has to come to terms with that failure.
For Carnage's character arc, you could make him initially just a mindless force of destruction. But as Spider - Man continuously thwarts his plans, he begins to develop a sort of grudging respect for the web - slinger. He might then start to target Spider - Man in more complex ways, not just brute - force attacks. This shows growth in his character from a simple villain to one with more depth. Also, you could have him face a situation where he has to choose between his own survival and the continuation of his chaos - creating ways, which could lead to an interesting internal conflict.