Well, one possible plot could be that Danny, while on an underwater mission to stop some ghostly threat near the ocean floor, gets hit by a strange spell or artifact that turns him into a mermaid. He then has to deal with his new form while also trying to keep his ghost - fighting duties going. Maybe he discovers new powers related to his mermaid form that help him in his battles.
Sam would probably be really intrigued. She might see it as a cool new form for Danny and would be eager to learn more about how it affects his powers. Tucker, on the other hand, would likely be more concerned about how to reverse it at first. But once he realizes that Danny can still do some cool things as a mermaid, he'd be excited too.
One common theme is the clash of two different worlds. Danny comes from a world of ghosts and humans, and mermaids bring in their own underwater realm. So there's often a theme of how these worlds interact and collide. For example, in some fanfictions, Danny has to protect the mermaids from ghost threats that start to encroach on their territory.
One idea could be that Danny turns into a girl due to a ghostly spell gone wrong. In the fanfiction, he could struggle with the new body while still having to fight ghosts. Maybe his friends don't recognize him at first, leading to some comical and tense situations.
There's this fanfiction called 'Danny Phantom and the Mermaid's Quest'. In it, Danny gets involved in a quest that is somehow related to mermaids. It has a lot of action and adventure scenes as Danny tries to figure out what the mermaids need from him. And 'The Mermaid's Allure in Danny's World' is also a great read. It focuses more on the relationship between Danny and a particular mermaid character.
Perhaps in this fanfiction, Danny as a kid would have a different perspective on the ghost world. He might be more curious and less jaded. His appearance as a kid could also change how the ghosts interact with him. Some ghosts might underestimate him, which could be an advantage for Danny. He could use this to his benefit when dealing with them, while also trying to figure out how to turn back to his normal self.
Perhaps there could be a love interest that is unique to his female form. Maybe someone who didn't notice Danny before as a boy, but now is attracted to him as a girl. This could add a whole new layer of drama and romance to the fanfiction, as Danny has to figure out how to handle this new relationship while still dealing with his ghost - hunting duties.
Well, one possibility could be that Danny gets hit by some sort of ghostly ray that de - ages him. His friends, Sam and Tucker, would then have to take care of him while trying to figure out how to reverse the effect. They might face challenges like hiding him from his parents, who would be really confused if they saw their teenage son turned into a toddler. Also, Danny's ghost powers could be all out of whack in his toddler form, causing some unexpected chaos around town.
One possible plot could be that Danny Phantom, while exploring some sort of interdimensional rift, gets zapped by a strange energy that transforms him into a Pokemon. Maybe he becomes a Pikachu. He then has to figure out how to use his new Pokemon abilities to get back to his normal form. He might start by trying to communicate with other Pokemon, learning their ways and using his ghostly knowledge to his advantage in the Pokemon world.