Cersei Lannister would be an interesting character to explore. In the afterlife, she could be either trying to regain her power and status or facing the consequences of her actions. She might be in a constant battle with her own guilt and pride. And seeing how she interacts with other characters in the afterlife, especially those she wronged, like Margaery Tyrell, would make for a very engaging story.
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.
Definitely Tyrion Lannister. He was such a complex character in the show. In the afterlife fanfiction, he could be using his wits to outsmart the rules of the afterlife. He might be trying to find a way to make the afterlife a more just place, given how he was often treated unfairly in life. He could also be forming new relationships with characters he never interacted with before, like some of the ancient lords 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.
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.
I think Tyrion Lannister would be very interesting. He's already a clever character in Game of Thrones. In a fanfiction with Harry Potter, he could learn magic and use it in his devious plans. He might even go to Hogwarts to study magic and cause all kinds of chaos there.
Jon Snow would be very interesting too. If he gets a second chance, he could be more open about his true identity from the start. He could use his claim to the Iron Throne to bring about a more stable rule. Instead of being exiled again, he could rally the North and the other kingdoms behind him, and make decisions that would benefit all of Westeros without having to keep so many secrets.
It could be Jon Snow. If he has a Stand in the fanfiction, combined with his already proven leadership skills and combat abilities, he would be a force to be reckoned with. His ability to come back from the dead might also be related to his Stand powers in some way, making him even more formidable.
Definitely Jaime Lannister. His character had a lot of development in the original series. If reincarnated, he could either redeem himself even further or take a completely different path. He might try to make amends for his past actions by becoming a true knight, or he could turn against his family if he realizes the toxicity of their ways. His reincarnation would bring a whole new set of possibilities to the story.
I think Bran Stark could be very interesting too. As the Three - Eyed Raven, he has vast knowledge and powers. In a future fanfic, he could use his powers to foresee and prevent new disasters or to guide the future of Westeros in a more positive direction. However, his new powers might also make him a target for those who fear his influence.