A really interesting plot would be if the magic world had to deal with a technological invasion. Muggles start to develop technology that can detect magic in some way, and the wizarding world has to figure out how to adapt. In the 'harry potter carry on fanfic', Harry and his friends could be called in to help negotiate with the Muggle government while also trying to keep the magical world a secret.
Another plot could focus on the lives of the secondary characters from the original series. For example, Neville could be the head of Herbology at Hogwarts and discovers a new, dangerous plant that has some sort of connection to an ancient magic. The 'carry on fanfic' could then be about how he and his students deal with it, with some cameos from the main characters.
One idea could be exploring what would happen if Harry had a twin sister who was sorted into Slytherin. It would add a whole new dynamic to the story, with her having different views and friends than Harry. She could be caught between loyalty to her house and loyalty to her brother.
One idea could be that a dragon escapes from Gringotts and Harry has to use his skills to track it down. Maybe the dragon has a special connection to a powerful dark magic item that Voldemort was after.
One idea could be that Voldemort somehow enters the Runescape world to seek more power. Harry and his friends follow him there to stop him. They have to learn the rules of Runescape quickly to be able to fight Voldemort. Another idea is that a Runescape wizard discovers a portal to the Harry Potter world and brings some of the unique items from Runescape, like special runes, which causes a stir in the wizarding world.
A great plot for a Harry Potter academy fanfic might be a time - travel scenario. A group of students accidentally travel back in time to the founding days of Hogwarts. They have to navigate the different rules and attitudes of that era while also trying not to change history too much. Or perhaps there's a plot where a student is wrongly accused of a major crime at the academy and has to prove their innocence. They'll need to use their wits and the help of their friends to find the real culprit.
Perhaps in the fanfic, the snake is a key to unlocking an ancient magic that has been lost in the wizarding world. This could lead to a whole new adventure for Harry and his friends as they try to understand and use this magic.
One idea could be a fanfic where a character discovers a long - lost wand with a special power. For example, a wand that can open portals to different magical realms. The character then has to figure out how to use this power without getting into too much trouble with the Ministry of Magic.
One interesting plot idea could be exploring what would happen if the Sorting Hat made different decisions for the main characters. For example, if Harry was sorted into Slytherin instead of Gryffindor, his entire journey would be radically different. He might have had different friends, different enemies, and a different approach to dealing with Voldemort.
One idea could be a crossover where characters from both universes meet. For example, Harry Potter could encounter Eragon and they could learn from each other's magic systems. Maybe Harry could teach Eragon about some of the more complex spells in the wizarding world, while Eragon shows Harry how to bond with a dragon in a more in - depth way.
You could also create a plot where a character who thought they had no connection to the Potter heritage suddenly discovers they do. This revelation could change their entire life. They might have to deal with newfound magical abilities that come with the Potter bloodline, and also face the enemies that the Potter family has made over the years, like some die - hard Death Eaters who still hold grudges.
A great plot idea might be a Craftsmith who is actually a descendant of an ancient wizarding family with long - lost spells and abilities. This Craftsmith uses their skills to recreate those spells and in the process, uncovers a plot by dark wizards to use these forgotten magics for evil. As the story unfolds, the main characters have to stop the dark wizards while also learning about the new and old magics.