One of the great ones I've read is a crossover with 'Lord of the Rings'. It was really interesting to see how the different magic systems and cultures interacted. For example, the way the White Walkers were compared to the Ringwraiths was quite unique. The author did a great job of weaving the two complex worlds together without losing the essence of either 'Game of Thrones' or 'Lord of the Rings'.
I liked the 'Game of Thrones' and 'Harry Potter' crossover fanfiction. It had a really cool plot where the characters from 'Game of Thrones' had to deal with the magic of the wizarding world. Like, imagine Daenerys facing off against Voldemort. It was full of unexpected twists and turns. The fanfic also explored how the political intrigue in 'Game of Thrones' could play out in the context of Hogwarts and the Ministry of Magic.
One of the great 'Game of Thrones Fanfic' I've read is 'A Song of Ice and Fire' inspired fanfic that delved deep into the backstories of minor characters. It added a whole new layer of depth to the world created by George R. R. Martin.
One great 'Game of Thrones' fanfiction is 'The Dragon's Heir'. It offers an interesting take on what could have happened if a certain character had survived. Another good one is 'A Song of Ice and Fire Reborn' which weaves a complex new plot within the existing world of Westeros. There's also 'The North Remembers', a fanfic that focuses on the Starks and their journey in a different light.
Another interesting one is 'The Stark Restoration'. As the name implies, it's about the Starks regaining their power and honor in Westeros. It delves deep into the relationships between the Stark siblings and how they overcome the challenges in their path. It also has some great descriptions of the North.
One great 'Game of Thrones Vengeance Fanfiction' is 'The Vengeful Wolf'. It follows Arya Stark as she returns to Westeros with a new set of skills and a burning desire for vengeance against those who wronged her family. The story is full of action and stays true to the complex world of Game of Thrones.
I really liked 'The Red Woman and the Devil'. It's a fanfiction that brings together Melisandre from Game of Thrones and Lucifer. The way it explores their different views on magic and the afterlife is really interesting. Then there's 'Jon Snow and Lucifer: An Unlikely Alliance' which has an interesting take on how two very different characters from different universes could work together. And 'Daenerys in Lucifer's World' is great too. It shows how Daenerys would navigate a world full of modern - day devils and angels, using her wits and her dragons.
One great 'Game of Thrones' Varys fanfiction is 'The Spider's Web'. It delves deep into Varys' past and his motives in a really interesting way. It shows his early days in Essos and how he came to be the master of whispers in Westeros. The author does a wonderful job of staying true to his character as we know it from the show and books.
I liked the 'Games of Thrones' and 'Harry Potter' crossover fanfiction. In this, they brought the magic of Hogwarts into the world of Westeros. The wizards had to deal with the political intrigue and the harsh reality of the Seven Kingdoms. It was cool to see spells being used in battles against knights and the like. And the way the houses in 'Game of Thrones' reacted to this new form of power was really engaging to read about.
You can try fanfiction websites like Archive of Our Own (AO3) or FanFiction.net. AO3 has a large collection of fanfiction, and you can use the search function to look for 'Game of Thrones crossover' stories. FanFiction.net also has a lot of content, though the interface might be a bit different. Just type in the relevant keywords, and you'll likely find some interesting crossovers.
One popular crossover is 'Game of Thrones' with 'Lord of the Rings'. The epic fantasy worlds blend well together. You can see characters from both universes interacting in unique ways, like Aragorn meeting Jon Snow. Another is 'Game of Thrones' and 'Harry Potter'. Imagine the magic of Hogwarts in the brutal world of Westeros. It creates really interesting storylines.
A possible fanfiction could involve the creation of a new magic system in Westeros that combines the dark and mysterious nature of the Rinnegan with the existing magic in 'Game of Thrones'. For example, the White Walkers could be somehow related to the Rinnegan powers. Maybe the Night King was once a normal man who was experimented on with Rinnegan - inspired magic, which turned him into the powerful and terrifying being he is now. This would add a whole new layer of mystery and danger to the fight against the White Walkers.