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.
Another great pairing could be the Arbiter and Daenerys Targaryen. The Arbiter is a noble and respected figure in the Halo universe. Daenerys is a powerful queen. They could share their experiences of leadership and ruling. The Arbiter might be intrigued by Daenerys' dragons, and she could be interested in the Arbiter's technology and the way his people are organized. Their meeting could lead to a new era of cooperation between their two 'worlds' in the fanfiction.
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.
Charlotte and Trip would be quite the pair. Charlotte is the smart and level - headed friend in Henry Danger. Trip is the fun - loving and somewhat scatterbrained member of the Game Shakers crew. Their personalities are so different that it would create a lot of humorous and interesting interactions. Charlotte could help Trip focus on important tasks, while Trip could introduce Charlotte to a more laid - back and creative way of thinking.
One interesting pairing could be Jasper from Henry Danger and Kenzie from Game Shakers. Jasper is often the comical sidekick in Henry Danger, and Kenzie is really smart and business - minded in Game Shakers. Their interactions would be really funny as Jasper might do some silly things that Kenzie would have to correct or find a way to work around. At the same time, Jasper could bring out a more laid - back side of Kenzie, and they could develop a unique friendship while working on whatever adventure or project comes their way in the crossover.
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.
One interesting combination could be Cole from Infamous and Jon Snow. Cole has his powers and moral dilemmas, and Jon Snow is all about honor and leadership. Their meeting could lead to some great discussions about power and responsibility. Another would be Fetch from Infamous and Arya Stark. Fetch's edgy nature and Arya's vengeful streak could make for an exciting duo.
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.
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.