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.
I think Iron Man and Tyrion Lannister would be a really interesting pairing. Iron Man is this brash, genius billionaire, and Tyrion is the clever, outcast dwarf in Westeros. They could bounce ideas off each other, with Iron Man showing Tyrion his high - tech gadgets and Tyrion teaching Iron Man about the political games in Westeros.
Another interesting pairing would be AOC and Arya Stark. Arya is a fierce and independent fighter. AOC could inspire Arya to think about the bigger picture in terms of changing the social order. Meanwhile, Arya could protect AOC from the many enemies she would surely make in the cut - throat world of Game of Thrones as she tries to implement her ideas.
Definitely Clark Kent and Dean Winchester. Clark with his superpowers and strong moral code, and Dean with his tough - as - nails hunting skills and loyalty. They could have some great banter as they come from such different worlds but find common ground in protecting people. Another interesting pairing could be Sam Winchester and Chloe Sullivan. Sam's intelligence and knowledge of the supernatural could pair well with Chloe's investigative skills.
I think Jon Snow and Spock would make an interesting pair. Jon is a natural leader, and Spock's logical thinking could complement his more instinct - based leadership. They could learn a lot from each other, with Jon showing Spock about the harsh realities of a feudal society and Spock teaching Jon about the power of reason.
Jon Snow and Elsa seem to be a popular pairing. The idea of the brooding and noble Jon with the powerful and reserved Elsa has a certain appeal. Their different worlds and personalities could create an interesting dynamic.
Arya Stark and Morrigan. Arya is a skilled assassin in 'Game of Thrones' and Morrigan is a powerful witch in 'Dragon Age'. Their encounter could be really interesting. Arya might learn some new magical skills from Morrigan, while Morrigan could be intrigued by Arya's stealth and combat abilities.
Jon Snow and Daenerys Targaryen can make an interesting soulmate pairing in fanfiction. They both have noble hearts and are leaders in their own right. Their union could bring about a new era in Westeros. Another could be Arya Stark and Gendry. They have a history together, and Gendry has always had a soft spot for Arya, which could be developed into a soulmate - like relationship.
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.