Jaime Lannister and Klaus Mikaelson. Jaime has his own complex moral code and is a skilled swordsman. Klaus is a powerful and often - cruel original vampire. Their meeting could be explosive. Jaime might initially be disgusted by Klaus' actions, but Klaus could see potential in Jaime's fighting skills. They could either become bitter enemies or, in an unexpected turn, form a tenuous alliance if they realize they have common enemies or goals in this crossover world.
Arya Stark and Rebekah Mikaelson. Arya is a trained assassin, and Rebekah is a fierce vampire. They could learn a lot from each other. Arya could teach Rebekah some of her stealth and combat skills without the use of vampire powers, and Rebekah could introduce Arya to the world of the supernatural and its different rules of survival.
Tyrion Lannister and Marcel Gerard. Tyrion is known for his intelligence and political acumen, while Marcel is a savvy leader in the vampire world. They could form an unlikely alliance. Tyrion could help Marcel with his strategic planning against his vampire enemies, and Marcel could offer Tyrion protection in the dangerous world of 'The Originals' using his vampire army. Also, their banter would be really interesting as they are both quick - witted.
I think Ragnar and Tyrion would make an interesting pair. Ragnar is a fearless and shrewd leader, while Tyrion is intelligent but often underestimated. Their interactions could be full of banter and strategic discussions.
Arya Stark and Castiel. Arya is a skilled assassin with a thirst for revenge. Castiel is an angel who often struggles with understanding human emotions. Their interactions could be really fascinating. Castiel could help Arya understand the deeper spiritual side of her revenge, while Arya could teach Castiel about the importance of personal vendettas in the context of the cut - throat world of Westeros.
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.
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.
Harry Potter and Jon Snow would be a very interesting pair. Harry is used to fighting against dark magic and Jon is a natural leader who has faced many challenges in Westeros. They could learn a great deal from each other. Harry could teach Jon about magic and Jon could teach Harry about leading in a world without magic but full of political scheming.
Eragon and Jon Snow would make an interesting pair. They both have a sense of duty and a connection to something greater. Eragon has his dragon and his destiny as a dragon rider, while Jon Snow has his role in the Night's Watch and his claim to the Iron Throne. Their different leadership styles and moral compasses could lead to some great character development in a fanfiction.
Arya Stark and a young Viking warrior would be an interesting pair. Arya has her skills in stealth and combat, and the Viking warrior could have his own set of skills and values. They could learn from each other and go on adventures together.
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.
Definitely Cortana and Tyrion Lannister. Cortana is highly intelligent and has access to a vast amount of knowledge. Tyrion is known for his wits. They could form an alliance where Cortana provides strategic advice based on her analysis, and Tyrion uses his political acumen to make things happen. Their conversations would be really fascinating as they come from such different worlds but are both very clever.