One plot idea could be Fem Harry being a powerful sorceress in the Game of Thrones world. She could use her magic to influence the political landscape, perhaps helping a particular house gain an advantage. Another idea is that she gets caught in the middle of the battle for the Iron Throne, using her unique skills from the Harry Potter universe to survive and make her mark. For example, her knowledge of potions could be translated into creating powerful elixirs in the Game of Thrones setting.
Well, Fem Harry might be a dragon rider in the fanfiction. Since she has a natural inclination towards adventure and courage in the original Harry Potter story, it would be exciting to see her tame a dragon in the Game of Thrones world. She could then use the dragon to either protect the innocent or to assert her own claim to power. Also, she could form alliances with different characters, like Tyrion Lannister, and together they could come up with strategies to change the course of the power struggle.
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.
Another plot could involve the Death Eaters trying to take over Westeros. Voldemort sees the chaos in the Seven Kingdoms as an opportunity to expand his power. The characters from Game of Thrones have to learn about magic quickly to defend their lands against the dark wizards. For example, Jon Snow could be one of the first to realize the potential of magic and try to form an alliance with the wizards from Hogwarts.
One idea could be exploring what would happen if a minor character, like Gendry, had a more central role in the fight against the White Walkers. He could lead a group of common folk and prove that the power doesn't always lie with the nobles.
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.
Perhaps a fanfiction where Daenerys Targaryen lands in Westeros earlier and forms an alliance with the Starks before the events of the main series. This would completely shift the political landscape and the fight for the Iron Throne. She could bring her dragons to help fight the White Walkers from the start.
One idea could be that Oliver Queen somehow gets transported to Westeros. He could use his archery skills to survive in the dangerous world. Maybe he'd join a faction, like the Night's Watch, and teach them his modern combat techniques.
One idea could be exploring what would happen if a minor character, like Gendry, became the King of Westeros. He has a claim to the throne through his Baratheon blood and his time with the Brotherhood without Banners could give him a different perspective on ruling. Another plot could involve a female character, like Brienne, leading a new order of knights that are dedicated to protecting the realm in a more democratic way. Or perhaps a story where the White Walkers and the humans form an uneasy alliance against a new, even greater threat from Essos.
One idea could be the Hound's journey after leaving King's Landing. He could encounter a group of outcasts and form an unlikely alliance to protect a small village from raiders. It would show his growth from a bitter, angry man to a more empathetic protector.
In Spartan Game of Thrones fanfiction, a possible plot is the discovery of Spartan artifacts in Westeros. A young maester finds a hidden vault with Spartan weapons and armor. He tries to understand their purpose and power. Meanwhile, a knight from a minor house gets hold of one of the Spartan swords. With it, he gains an edge in combat and becomes a key player in the power struggles, attracting the attention of the major houses and changing the course of the political and military affairs in Westeros.
One idea could be a character from a minor house rising to great power through unexpected alliances. For example, a character from House Frey who tries to redeem the family's name after the Red Wedding by allying with the Starks and helping them take back Winterfell. This would bring a new dynamic to the story as the character has to face the prejudices against their house.