One plot idea could be that Dean, as a girl, has to deal with the different ways people treat her in the hunting world. Maybe some monsters underestimate her at first, but she quickly shows them she's still as tough as ever. Another idea is that she has to navigate her relationships with Sam and Castiel in a new way, especially any potential romantic feelings that might change or emerge because of her new form.
His core personality might not change much. He'd still be brave and protective. But he might be more in touch with his emotions, as society often allows women to be more openly emotional. He could also be more perceptive about the subtleties in relationships.
One idea could be that Dean and Elena meet when Dean is on a hunt in Mystic Falls. They initially clash because of their different approaches to dealing with the supernatural, but then they realize they can learn a lot from each other. Another plot could involve them teaming up to fight a common enemy that is a threat to both the world of Supernatural and The Vampire Diaries. For example, a powerful demon - vampire hybrid.
One idea could be a crossover where the magical world of Severus Snape and the hunting world of Dean Winchester collide. For example, Dean could end up in Hogwarts and Severus has to deal with this strange, non - magical yet very capable hunter in his midst.
A frequent theme is the reaction of his friends and family to his marriage. In many fanfictions, Sam has a big role in how he adjusts to Dean's new situation. Also, the idea of building a normal life in the midst of the chaos of the supernatural world is quite common.
One plot idea could be that Dean Winchester somehow gets transported into the wizarding world. He might team up with Harry Potter to hunt down a dark wizard who is using dark magic to summon demons. Dean's knowledge of the supernatural from his own world would be a great asset, and Harry could introduce him to the magic of spells and potions.
One idea could be that Dean, as a girl, has to navigate the same dangerous situations but with a different set of challenges due to his new gender. For example, he might face different reactions from male characters who underestimated him before. Another could be exploring his relationships, like how his friendship with Sam might change, with Sam having to protect 'her' in a different way. And perhaps there could be a plot where 'female Dean' has to use her newfound charm to get information in a case in a way that male Dean couldn't have.
Well, perhaps the oc is cursed by an evil witch to be turned into a kitten as a form of punishment. In this case, the oc has to go on a quest to find a rare magical flower that can break the curse. Or, the oc could be part of a scientific experiment that goes haywire and turns them into a kitten. They then have to escape the laboratory and find someone who can understand their situation and help them change back. Also, it could be that a jealous rival uses a powerful talisman to transform the oc into a kitten, and the oc must outwit the rival to regain their human form.
One interesting plot point could be how Hiccup adjusts to the new physical form. For example, he might struggle with the different clothing styles and social expectations for girls in his world. Another could be the reactions of his friends. Some might be shocked at first, while others could be more accepting and help him through the transition. There could also be a plot point about how this change affects his relationship with Toothless. Maybe Toothless initially doesn't recognize him and Hiccup has to find new ways to bond with his dragon.
Perhaps a plot where Dean is pregnant and barefoot in a post - apocalyptic world. He has to survive not only for himself but also for the baby. His skills as a hunter are put to the test in a different way as he has to find food and shelter suitable for his condition. He might also face challenges from other survivors who either want to harm him or take advantage of his situation.
One idea could be that Superboy is hit by a strange alien device that changes his gender. He then has to deal with the new physical limitations and social expectations while still trying to be a hero. For example, he might struggle with the different way people treat him as a girl and how to use his powers in a more feminine body.