One of the most interesting combinations would be Jon Snow and the Ranger from 'Shadow of War'. Jon Snow is a natural leader and a great warrior in 'Game of Thrones'. The Ranger in 'Shadow of War' has a deep understanding of the dark forces at play in his world. If they joined forces, they could lead an army against the common threats like the White Walkers in 'Game of Thrones' and Sauron's forces in 'Shadow of War'. They could exchange combat strategies and leadership skills, and perhaps Jon Snow could even learn something about the supernatural powers that the Ranger has at his disposal.
Daenerys Targaryen and Rosalie Hale would be an exciting combination. Daenerys is a strong and powerful queen, and Rosalie is a fierce and beautiful vampire. Rosalie could teach Daenerys some of her vampire - like self - defense skills, and Daenerys could show Rosalie how to rule with more confidence and grace. They could also form an alliance based on their mutual desire for power and respect.
Arya Stark would be a great choice. You could write about her using her assassin skills to target key military leaders during the war. For example, she could infiltrate enemy camps disguised as someone else and take out important commanders, which would have a huge impact on the battles.
Arya Stark would be really interesting. She has the skills of a trained assassin. In the Fallout world, she could use her stealth and combat abilities to navigate the dangerous wastelands and take on raiders or other threats. Her ability to change her face could also come in handy for undercover operations or infiltrating enemy camps.
Talion would be a very interesting character to crossover. His skills as a ranger and his connection with the wraith are quite unique. In the Worm universe, he could use his combat abilities and stealth to great effect. He might be seen as a wildcard by the different factions in Worm. His sense of justice, developed in Shadow of War, could also put him at odds with some of the more villainous characters in Worm.
Clark Kent and Arya Stark would be an interesting pairing. Clark with his superpowers and noble nature, and Arya with her assassin skills and strong will. They could learn a lot from each other. Clark could teach Arya about using her powers (if she were to gain some in this fanfic) for good, and Arya could teach Clark some of her stealth and combat techniques.
Reincarnating as Tyrion Lannister would be really interesting. He's already a complex character with a sharp mind. As someone reincarnated, you could use your modern ideas to enhance his political acumen even more. You could also try to change his relationship with his family for the better, or at least make his life less miserable while still being the clever dwarf everyone fears and respects in the political arena.
Black Widow. She's a master spy and assassin. In the world of Game of Thrones, she could easily blend in with the various spies and informants. She could play the game of thrones better than most of the characters there, using her charm and combat skills to manipulate the situation to her advantage. She might even be able to uncover some of the deepest secrets of the Seven Kingdoms.
Jon Snow would also be a great addition. His journey from being a bastard to a leader in Game of Thrones could be paralleled in the world of Frozen. He could learn from Elsa about controlling his powers (if we assume he has some new ice - related powers in this crossover) and in turn, teach the people of Arendelle about loyalty and sacrifice, which are values he has held dear in Westeros.
I think Arya Stark would be really interesting. In life, she was a fierce assassin. In the afterlife, she could use her skills in different ways. Maybe she's on a mission to hunt down the spirits of those who wronged her family. Or she could be teaching other souls self - defense in this new realm.
Arya Stark in a 'Naruto Game of Thrones crossover fanfiction' would be fascinating. If she were to learn some of Naruto's ninja skills, she would become an even deadlier assassin. And Naruto's influence might change her path a bit, making her less vengeful and more about bringing real change to Westeros.