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.
Jon Snow is always interesting. In fanfiction, you could explore his leadership during a war against the White Walkers in more detail. Maybe he has to make some really tough decisions, like sacrificing a small part of his army to save a larger group or making deals with former enemies to get more resources for the fight.
Theon Greyjoy also has potential. You could write about his redemption arc during the war. He could start off trying to regain his family's trust by doing some underhanded things for the Iron Islands but then have a change of heart and end up fighting for the Starks against his own family in a big, dramatic battle.
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.
One of the most interesting combinations would be Jon Snow and the Ranger from 'Shadow of War'. Jon Snow is a natural leader and a great warrior in 'Game of Thrones'. The Ranger in 'Shadow of War' has a deep understanding of the dark forces at play in his world. If they joined forces, they could lead an army against the common threats like the White Walkers in 'Game of Thrones' and Sauron's forces in 'Shadow of War'. They could exchange combat strategies and leadership skills, and perhaps Jon Snow could even learn something about the supernatural powers that the Ranger has at his disposal.
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.
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.
Spider - Man is always an interesting choice. He was caught in the middle of the Civil War in the original story. You could explore his internal struggle in more depth. He has the loyalty to Tony Stark who he sees as a mentor, but also believes in Captain America's ideals. His actions could be the key to either making the situation worse or better. For example, if he made a different choice than in the original story, it could change the entire course of the Civil War.
Loki in a cartoon Infinity War fanfiction could be a fascinating focus. He's so full of mischief and trickery. You could show his internal struggle between his desire for power and his loyalty (or lack thereof) to his brother Thor in a more cartoony, over - the - top way. Maybe he teams up with the heroes against Thanos in the most unexpected way, all while plotting his own schemes on the side.
Theon Greyjoy is a great choice. His complex and tortured past can be further explored in a dark fanfiction. You could write about his internal struggle between loyalty to the Starks and his own family. Another interesting character is Melisandre. Her mysterious powers and motives can be delved into deeper, perhaps she has a more sinister plan all along. And of course, Ramsay Bolton. His cruelty and madness can be taken to new levels in a fanfiction.
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.