Jon Snow would be really interesting. He's already a complex character with a mysterious background. Seeing him travel to other dimensions could explore his identity further. He might discover more about his true parentage in a different dimension or gain new skills that could help him in his fight for the Iron Throne in his own world.
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.
Daenerys Targaryen. She has a strong will and a mission to claim the Iron Throne. If she time - traveled, she could meet her ancestors and learn from their successes and failures. She could also try to prevent the downfall of her family in the past, which would have a huge impact on the present state of Westeros in the fanfiction world.
Arya Stark would be really interesting. She already has assassin - like skills in the original GoT. In a crossover, she could learn more advanced techniques from the Assassins' Creed world and become an even more formidable force.
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.