One idea could be that Slender Man somehow gets transported into the Simpsons' world. He might start lurking around Springfield Elementary, causing the kids, including Bart and Lisa, to feel an unexplained sense of dread. Another plot could involve Homer mistaking Slender Man for a really tall, creepy new neighbor and trying to befriend him in his typical, bumbling way.
A more complex plot could be that Slender Man is actually a manifestation of some deep - seated fear within the Springfield community. The fanfiction could explore how the different characters deal with this collective fear. For example, Mr. Burns might try to use Slender Man for his own evil purposes, while the Simpson family tries to find a way to banish him back to wherever he came from.
Perhaps Slender Man targets Marge's famous blue hair. He could be drawn to it because it's unique in his strange, otherworldly eyes. This would set off a chain of events where Marge has to protect her hair and her family from this menacing figure. The family could then try to enlist the help of Ned Flanders, who might think it's a test from God.
Bart would probably be both scared and curious. He might initially try to play pranks on Slender Man, but as things got scarier, he'd realize he was in over his head. Lisa, on the other hand, would likely try to research Slender Man to find a scientific or logical explanation for his existence.
A possible plot idea is that Slender Man is a creation of an ancient and dark experiment in the RWBY world that went wrong long ago. Now he has re - emerged and is seeking revenge on those who originally created him. The RWBY team gets involved when they discover that his actions are endangering the safety of the entire world. They have to battle not only his physical form but also the psychological fear he instills in people. Slender Man could also have the ability to manipulate the nightmares of the RWBY characters, which adds a whole new level of horror and psychological thriller elements to the fanfiction.
Maybe in the fanfiction, Slender Man has an alliance with Orochimaru. Orochimaru is always interested in strange and powerful beings. Together, they plan to take over the ninja world. Naruto and his friends then have to not only face Orochimaru's usual schemes but also deal with the terrifying abilities of Slender Man.
One idea could be exploring what would happen if Homer got a different job, like becoming a famous artist. It would be fun to see how his new career would change the family dynamics.
Well, a great Simpsons fanfiction concept might be centered around Mr. Burns getting amnesia and forgetting he's rich. He then starts living a normal life and has to learn about kindness and friendship from the other Springfield residents. Also, you could write a fanfic where Nelson actually has a soft side and befriends Lisa, and they have adventures together. And there could be a story where Krusty the Clown tries to retire but the town won't let him because he's such an important part of Springfield's culture.
One idea could be a crossover with 'Futurama'. The Simpsons characters could travel to the future and interact with Fry, Bender and the rest. Maybe Homer gets a job at Planet Express and causes all kinds of chaos with his clumsiness. Another could be a crossover with 'South Park'. The two sets of very different characters would create a really comical and chaotic situation. For example, Bart could get into a prank war with Cartman.
One idea could be creating an OC who is a new neighbor in Springfield. They could have a really unique hobby, like collecting antique typewriters. This OC could then interact with the Simpson family in various ways, like getting into a friendly competition with Homer over a barbecue recipe.
Well, perhaps there could be a fanfic where Slender Man is actually an ancient evil that was sealed away in the RWBY world long ago. And due to some recent events, like the opening of a certain relic or a powerful magic spell gone wrong, he is released. The RWBY characters then have to figure out his weaknesses by researching old legends. Or it could be that Slender Man has a connection to the White Fang. He uses their anger and desperation to his advantage, secretly manipulating them to do his bidding while the RWBY gang tries to stop this new threat.
One interesting story could be about a group of kids who stumble upon the Slender Man's forest domain. They start experiencing strange hallucinations and find themselves being hunted by the faceless entity. It's full of suspense as they try to find a way out of the forest while constantly feeling his presence.
One interesting Simpsons fanfic idea could be a story where Homer discovers a hidden talent for painting and becomes a famous artist in Springfield. Another could be about Lisa starting a new club at school that promotes environmental awareness in a really creative way. And a third idea might be a fanfic where Bart travels to the future and sees how Springfield has changed.