One interesting storyline could be about an alternate version of Sam and Dean where they are vampires instead of hunters. It would completely flip their roles and moral compasses.
Many fanfictions focus on the relationships between the characters in new ways. Like a story where Dean and Crowley form an unexpected alliance that goes beyond their usual enmity. They might have to work together to defeat a common enemy that is more powerful than anything they've faced before, and in the process, they develop a strange kind of friendship.
One interesting storyline could be exploring the backstories of the minor characters in the Masters of the Universe universe. For example, delving deeper into the origin of Orko, his life on his home planet before coming to Eternia. Another could be a story where He - Man and Skeletor have to team up against a greater evil that threatens both their interests. This would flip their usual dynamic on its head.
One interesting storyline could be a new hero emerging from a small town. This hero has a unique power that the Avengers initially mistake for a threat. So, there's a lot of tension and action as the Avengers try to figure out if this person is friend or foe. Eventually, they discover the hero's good intentions and they all team up to face a greater threat like an alien invasion.
One interesting storyline could be a Nephilim trying to hide its true identity from both angels and demons while living among humans. It would face the struggle of controlling its powers and not being discovered.
One interesting storyline could be about the benders facing a new and powerful supernatural entity that challenges their abilities in unexpected ways. For example, they might encounter a spirit that can manipulate time, causing chaos in their world.
Well, in some Supernatural paralyzed fanfiction, there might be a story where a beloved character is paralyzed as a result of a spell gone wrong. Their friends in the Supernatural world might search for rare ingredients to create a cure. It could also involve the character having to adapt to their new state and still contribute to the fight against evil in some way, like using their knowledge of the occult to guide the others even while being paralyzed.
Well, in some fanfictions, the Queen of Hell might be involved in a love triangle with a main character from Supernatural and another supernatural being. This could lead to all sorts of drama and conflict, as her power and influence would surely disrupt the normal relationships in the story. Also, there could be a plot where she tries to rewrite the rules of Hell, causing chaos both in Hell and on Earth.
In some fanfictions, there might be a situation where the Flash's speed force is somehow connected to the supernatural world in Supernatural. Maybe it attracts the attention of angels or demons who want to use it for their own purposes. This could lead to a complex plot where the Flash has to deal with both his own villains and the otherworldly beings from Supernatural.
One interesting storyline could be where Steven's gem powers interact with the magic in the Undertale world. For example, his ability to heal might have a unique effect on the monsters in Undertale. Maybe he could heal some of the more damaged monsters and in turn, they help him understand their world better. Another could be a story where the characters from both universes team up to face a common threat. The Sans and Papyrus from Undertale could join forces with Steven and the Crystal Gems to fight against a powerful enemy that is a threat to both their worlds. It would be really cool to see how their different fighting styles and powers work together.
One interesting storyline could be about a new and powerful character emerging in Universe 9. This character might have a unique ability that challenges the existing power hierarchy. For example, they could control a type of energy not seen before in the Dragon Ball universe.
A really cool storyline in a Naruto - DC crossover fanfiction could be the idea of Naruto characters training DC heroes in the ways of chakra. Superman, for example, could learn to control chakra and use it to enhance his already superhuman abilities. Or, a story where the Justice League has to deal with the political intrigue of the Hidden Villages in Naruto's world. It would be a clash of two very different political systems. Also, a storyline where a Naruto character like Sasuke becomes a sort of anti - hero in the DC Universe, causing chaos among the heroes and villains alike could be fascinating.