One idea could be Harry Potter arriving in Skyrim and using his magic to combat the dragons. Maybe he teams up with the local mages and they learn from each other's magical techniques.
A possible plot could involve a Dragonborn who discovers a hidden portal to the wizarding world. In the wizarding world, they learn about magic wands and spells. The Dragonborn could then combine their Thu'um powers with magic spells to fight against dark forces that are threatening both worlds. For example, Voldemort could team up with Alduin in some way, and the Dragonborn and Harry Potter have to work together to stop them.
Maybe Harry gets involved in the civil war in Skyrim. He could use his Hogwarts-taught dueling skills to fight in battles, and his knowledge of magic could be used to solve some of the political intrigues. He might also try to find a way to bring peace to Skyrim using his unique perspective on magic.
One idea could be that Hogwarts is magically transported to Skyrim. The students have to deal with the cold, the dragons, and the different magic systems. Another could be that Harry and his friends are sent on a quest in Skyrim to find a powerful magical artifact that could help defeat Voldemort. Maybe they have to team up with the Nords and learn their ways of fighting and magic.
One interesting concept could be the combination of magic systems. In Harry Potter, there's wand - based magic, while Skyrim has shouts and spellcasting in a different way. A fanfic might explore how wizards from Hogwarts would adapt to the magic in Skyrim, or vice versa. For example, a Hogwarts student trying to master a dragon shout.
One idea could be exploring what would happen if a new, mysterious student joined Hogwarts during the Triwizard Tournament. This student could have a hidden agenda related to the Dark Arts and start influencing the events of the tournament in unexpected ways.
One interesting plot idea could be Harry's return to Hogwarts as a teacher. He could pass on his unique experiences and knowledge to the new generation of students.
One idea could be a dragon that forms a bond with Harry and helps him in his battles against Voldemort. Maybe the dragon has some special powers that are crucial for Harry to win.
One idea could be Harry and his friends exploring new magical realms that were previously unknown. Maybe they discover a hidden school of magic that has a different approach to teaching spells.
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.
Well, a great plot for a Harry Potter mansion fanfic could be centered around a lost heirloom. The mansion was once owned by an important wizarding family, and a valuable heirloom has been lost within its many rooms and corridors. The characters embark on a quest to find it, facing various challenges like magical traps and puzzles. Also, the mansion could be a place where time behaves differently. People who enter might find themselves in different eras of the wizarding world, and they have to find their way back to the present while also uncovering the secrets of the mansion's past.