Perhaps a plot where Sonic and Spider - Man swap powers. Sonic has to learn how to use Spider - Man's webs and wall - crawling abilities while Spider - Man has to deal with Sonic's super speed. This would lead to a lot of comical and action - packed situations as they try to navigate their new - found abilities in their respective worlds.
A plot idea could be that an evil scientist has created a device that combines the powers of both Sonic's chaos emeralds and Spider - Man's radioactive spider - bite. This new power source is unstable and threatens to destroy both their universes. Sonic and Spider - Man must work together to stop the device. It's interesting because it forces them to understand each other's powers and find a way to neutralize this new threat.
One interesting plot could be that a villain from Sonic's world has found a way to travel to Spider - Man's universe and is causing chaos there. Sonic follows the villain and teams up with Spider - Man. Along the way, they discover that the villain's plan is much bigger than they thought and it involves using the unique aspects of both universes to create a new, dark world. They have to put aside their differences and use their combined skills to stop this from happening.
One idea could be Sonic having to deal with his new vampire abilities while still trying to protect his friends from the usual villains. For example, he might struggle with the urge to drink blood but not want to harm his friends. Another plot could involve Sonic trying to find a cure for his vampirism while on the run from vampire hunters who mistake him for an evil creature.
One idea could be Silver coming from the future to prevent a new threat in the Sonic X universe that the others are unaware of. He might team up with Sonic and his friends in unexpected ways, like using his psychic powers to help them access hidden areas. For example, he could use his powers to levitate Sonic over a large chasm that the others can't cross.
One idea could be Sonic having to face a new and more powerful dark version of the Black Knight. This would test his skills and force him to find new ways to use his speed.
One interesting idea could be creating an OC that has a unique power related to time manipulation. For example, they can slow down time to gain an advantage in races or battles against Sonic and his friends. Another idea is an OC from a different dimension who comes to Sonic's world with strange technology that no one has seen before. It could lead to interesting interactions as Sonic and his friends try to understand this new arrival.
A plot where Spider - Man teams up with a character like Arya Stark could be really cool. He could teach her some of his combat moves, and she could teach him about the ways of survival in Westeros. Together, they could take on the villains in both their worlds, like the Lannisters and some of Spider - Man's classic foes who might also find their way into this crossover world.
One interesting plot idea is Sonic being mistaken for a new superhero by the Avengers. He has to prove himself and his abilities to them. Another is when the Avengers' world is invaded by Sonic's enemies, and they have to work together to defeat them.
One idea could be that the OC has a unique power that Sonic needs to save the day. For example, the OC might be able to control time briefly, and Sonic teams up with the OC when Dr. Eggman creates a time - bending weapon. Another idea is that the OC is from a different dimension and Sonic has to help the OC get back home while facing various challenges together. Maybe the OC has a special charm that attracts all the little critters in Sonic's world, and this causes some comical situations as Sonic tries to keep things in order while protecting the OC from Eggman's clutches.
One great Sonic the Hedgehog and Spider - Man fanfiction could be where they team up to stop a common enemy that has the ability to manipulate time. In this story, Sonic's speed and Spider - Man's agility and web - slinging skills would make for an exciting combination as they race against the clock to save the world.
One idea could be exploring the different Spider - People's home universes in more detail. For example, showing what Miles Morales' daily life in his universe was like before becoming Spider - Man. Another could be a team - up between Miles and an older, more experienced Spider - Person from a different universe to face a new and powerful villain.
Perhaps there's a fanfiction where Spider Man and Spider Gwen accidentally swap powers. They have to learn how to use each other's abilities to fight crime. This would lead to some really funny and also challenging situations as they get used to different ways of web - slinging and sensing danger.