Daenerys Targaryen is another option. A transmigrator in her place would inherit her dragons and her claim to the Iron Throne. They would have to navigate her journey across the Narrow Sea, deal with the challenges of ruling the cities she conquers, and decide how to approach the political situation in Westeros. It would be fascinating to see how a different personality would handle her powerful position and her destiny.
Jaime Lannister would also be a great character for transmigration. His character arc is so complex, and a transmigrator in his body would have to grapple with his reputation as the 'Kingslayer' and his changing loyalties. They could potentially change the course of his relationship with Cersei and his role in the power struggles of Westeros.
Arya Stark could be an interesting choice. Imagine if someone else transmigrated into her body. They would have to deal with her complex relationships, like her hatred for the Lannisters and her training with the Faceless Men. It would be a challenge to continue her story while also bringing in new elements from the transmigrator's personality or knowledge.
Arya Stark would be a great choice. You could write about her using her assassin skills to target key military leaders during the war. For example, she could infiltrate enemy camps disguised as someone else and take out important commanders, which would have a huge impact on the battles.
Definitely Tyrion Lannister. In a crossover with 'The Simpsons', he could bring his sharp wit and political acumen to Springfield. He'd probably outwit Mr. Burns in no time. His complex personality and ability to adapt would make for a very engaging character in such a crossover fanfiction.
Arya Stark would be a great choice. She's already a skilled fighter in the ways of the Faceless Men. Training with samurai could enhance her skills even further. Her journey of self - discovery could be influenced by samurai teachings about honor and discipline.
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.
Jon Snow would be an interesting character to be an avatar. Given his complex backstory and his role in the fight against the White Walkers, as an avatar he could gain even more powerful abilities. Maybe he could have the power to control the elements, which would be a huge advantage in battles. He could use this to create blizzards against the White Walkers or to put out fires during the sack of King's Landing.
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.