In this fanfiction, maybe Dean forgets he is human due to a magical spell. He then starts to act more like a wild animal, relying on instinct rather than reason. His brother Sam would be desperate to reverse the situation. He might search through old books and consult with other hunters to find a cure. Meanwhile, Dean could be getting into all sorts of trouble as he doesn't recognize the danger of certain situations that a human would.
One possible plot could be that Dean suddenly wakes up one day with no memory of his humanity. He might start exhibiting strange behaviors, like not understanding emotions or basic human needs. For example, he could be confused by the concept of hunger or sleep. His friends would then have to find a way to help him remember who he really is, perhaps by taking him to places that hold significant memories for him.
Perhaps an ancient and malevolent spirit. This spirit might have been awakened by the Winchesters' activities and decides to target Dean. By making him forget his humanity, the spirit hopes to create chaos and destruction. The spirit could be using Dean as a pawn in its grand scheme to cause mayhem in the world of the living.
Maybe in the fanfiction, Voldemort devises a plan where Snape forgets Harry to disrupt the Order of the Phoenix's plans. Without Snape's knowledge of Harry and his role in the prophecy, things start to go awry. Harry has to find a way to make Snape remember, perhaps by going on a quest to find a magical relic that can restore memories. Hermione and Ron would be by his side, of course, providing their usual support and knowledge.
One possible plot could be that Naruto suffers a head injury during a battle. As a result, he forgets about Sasuke completely. His friends initially don't notice, but as time passes, they realize Naruto's behavior towards Sasuke - related things is strange. For example, he doesn't get angry when Sasuke is mentioned like he used to. Then they try to help Naruto regain his memories of Sasuke through various means, like showing him old photos or taking him to places they used to go together.
One possible plot twist could be that Naruto's memory loss leads to him accidentally endangering his former teammates. For example, he might unknowingly reveal secrets to the wrong people. Another could be that his forgotten bond with Team 7 causes him to form new alliances that are initially at odds with his old team. Maybe he teams up with former enemies who manipulate his lack of memory. Also, it could lead to a situation where Team 7 has to reteach Naruto about their shared past and values, which would be a really emotional and character - developing journey for all of them.
Well, it could be that Dean stumbles upon a baby in some unexpected place like an abandoned building. He then has to figure out what to do with the baby, whether to find its parents or take care of it himself for a while.
One way the plot could develop is that Naruto begins to trust the wrong people because he doesn't remember who he should trust. This could lead to him getting into dangerous situations. Meanwhile, his true friends are desperately searching for a way to reverse his memory loss. As the story progresses, Naruto might start to have flashes of his old memories, which would add some mystery and hope to the plot.
Well, it could start with Ginny waiting all day for Harry to remember. When he doesn't, she gets angry and ignores him. Harry, being a bit clueless, doesn't understand why she's so mad until Hermione smacks him on the head (figuratively) and tells him. Then he might plan a surprise party in the Room of Requirement with all of Ginny's favorite things, like butterbeer and Weasley sweaters in different colors.
Maybe Dean gets cursed by a witch. The tail could be a side - effect of the curse that slowly changes him. He has to find a way to break the curse while still hunting monsters, and Sam has to deal with his brother's new 'accessory' and the problems it causes during their hunts.
In this 'dean becomes a dragon supernatural fanfiction', it could start with Dean being in a fight with a powerful demon. During the battle, the demon casts a spell that backfires and instead of killing Dean, it turns him into a dragon. Dean then has to deal with the fact that he is no longer fully human. He may have to hide from other hunters who might see him as a threat. Meanwhile, he tries to figure out how to use his new dragon abilities to continue his fight against the supernatural, all while trying to find a way to turn back to his normal self.
One possible plot twist could be that Harry discovers some hidden magical ability within himself that is related to his new family with Dean. Maybe it's a unique type of magic that only his new family line has.