One idea could be exploring the daily lives of the characters who stay at Shell Cottage. For example, showing how they heal and recover from their battles. Another could be a story about the hidden secrets of Shell Cottage that are slowly uncovered by the characters. Maybe there are magic spells or artifacts hidden there.
One interesting plotline could be a detailed exploration of the post - battle recovery at Shell Cottage. Maybe characters dealing with the emotional trauma while staying there. For example, Hermione helping to heal Ron's more hidden wounds from the battle.
One possible fanfiction could be about a character finding a magical lemon tree near Shell Cottage that has special healing properties. The lemon's essence is used to cure a minor ailment of one of the main characters.
Obviously, Bill and Fleur could be main characters since Shell Cottage is their home. They could be dealing with the post - war situation, like how to protect their home and family from any remaining threats.
A plotline could involve the 'cucklord' having some sort of hidden agenda related to Shell Cottage's location or the magic it holds. They could be trying to use it for their own gain, like accessing a powerful magical artifact that is hidden there. They might manipulate the characters living in the cottage to achieve their goals, creating a lot of tension and drama as the other characters try to figure out what's going on.
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.
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.