If we're talking about a fanfiction about Joffrey's younger brother in a more general sense. Imagine if he was born with a special ability, like being able to communicate with animals. In a world as cutthroat as Game of Thrones, this could be both a great advantage and a huge danger. He might use his ability to gather intelligence, but at the same time, others might see him as a freak or a threat. This could lead to all sorts of adventures and political intrigue as he tries to navigate the world of Westeros.
Well, if it's about Myrcella, Joffrey's sister (but still in the family dynamic), she could be kidnapped not for political reasons as in the show but by a group who wants to use her to expose the incestuous nature of the Lannister family. She then has to decide whether to go along with it or try to protect her family's secrets while also trying to survive. This could create a really complex and morally ambiguous situation.
One possible plot could be that Joffrey's twin brother is the complete opposite in nature. While Joffrey is cruel and tyrannical, his twin is kind - hearted and wise. He could secretly try to undo the damage Joffrey does, like secretly helping the common people that Joffrey oppresses. He might also try to influence his mother, Cersei, to be more merciful, which would create a lot of internal family drama as Cersei is so protective of Joffrey.
One possible fanfiction could be about Joffrey's brother Tommen growing into a wise and just ruler. He could be shown learning from the mistakes of his older brother. He might start secretly training in combat and political strategy, with the help of some of the more noble characters in the series, like Tyrion perhaps. He could then start to make changes in King's Landing, trying to reverse the chaos that Joffrey had caused.
There could be a fanfic where a character from the series, say Arya Stark, returns to Harrenhal in a different time or situation. Maybe she has to face an old enemy who has taken refuge in the castle. This would create an exciting plot as she uses her skills and knowledge of the place to her advantage. Harrenhal is a perfect setting for a story full of intrigue and action.
One idea could be an OC who is a healer in King's Landing during Joffrey's reign. The OC secretly tries to undermine Joffrey's cruel rule by using their medical knowledge to help those he persecutes. They might start by treating the families of those unjustly imprisoned. This could lead to a dangerous game of cat and mouse with Joffrey and his cronies as they suspect someone is working against them.
The plot could be about his search for justice. His father was a great knight, and he feels the need to right the wrongs in the world. Maybe he discovers a conspiracy related to his father's death and sets out to expose it.
One interesting scenario could be that it was actually a planned assassination by a secret alliance of the Starks and the Tyrells long before the wedding. They worked together in the shadows, using Sansa as an unwitting pawn. The Tyrells provided the poison, and the Starks had someone in place at the wedding to ensure Joffrey drank it.
Some fanfictions might focus on an alternate universe where Joffrey is not a king. He could be a simple knight, and the story could explore how his personality would develop without the power and privilege of the throne. He might learn humility and honor in a different way.
One interesting scenario could be a rewrite of their relationship where Joffrey is secretly influenced by a more benevolent force. This might lead him to treat Sansa with more kindness than in the original series. Maybe he starts to see the value in having an ally in Sansa rather than just using her as a pawn.
One idea could be exploring what would happen if a minor character like Gendry had been secretly trained in magic. He could use his newfound powers to unite the smallfolk against the warring nobles. Another plot could revolve around a female knight from the North who disguises herself as a man to fight in the wars, and along the way, she discovers secrets about the White Walkers that no one else knows. Or perhaps a fanfiction where Daenerys and Sansa form an alliance much earlier and together they reshape the power dynamics of Westeros.
A really cool 'Game of Thrones fanfiction' plot could involve the resurrection of some of the dead characters in a different way. For example, what if Ned Stark was brought back not as a vengeful wraith but as a wise advisor to a new king or queen? His honor and wisdom could shape the future of Westeros in a completely new direction. And it would be interesting to see how the other characters react to his return.