It could be a new female character. Maybe a commoner who rises to power by her own determination. She could be from a small village and through her skills, like her ability to read and write, which is rare for commoners in the 'Game of Thrones' world, she catches the attention of the nobles and starts to make a name for herself in the political arena.
Perhaps it's a long - lost relative of one of the major houses. By her hand, she tries to reclaim the glory of her family. For example, if she is related to House Stark, she might be on a mission to bring back the honor and power of the Starks after they have faced so many hardships. She would use her own unique skills and knowledge to do so, different from the typical Stark way of fighting and ruling.
A Skagosian outcast. He could have been banished from his tribe for some reason, like not following their brutal traditions. This character would be interesting as he would be an outsider even in his own land at first, and his journey to either regain his place or find a new one could be full of adventure and self - discovery.
Bran Stark could also be the main character. His new powers as the Three - Eyed Raven give him a whole new perspective. He could be the one to guide Westeros into a new era. He could use his knowledge of the past and his ability to see things that others can't to make decisions that would shape the future of the Seven Kingdoms.
Melisandre could be a main character if you consider her as a kind of warlock. She has her strange powers, like seeing the future in the flames. You could write a fanfiction exploring her past, how she became a red priestess and gained her powers, and her further adventures in Westeros, perhaps with her using her powers in different ways than in the original series.
Arya Stark could be an interesting main character as a summoner. She already has a lot of skills like stealth and combat. If she were a summoner, she could summon the spirits of those she has killed to either help her in battles or to gather intelligence. It would be a new twist on her character.
It could be a young apprentice wizard who has just discovered his powers. He is eager to prove himself and protect the North from all the threats it faces.
Perhaps a young knight who is given a lordship as a reward for his heroism in a great battle. He is inexperienced in ruling but has a strong sense of justice. He has to learn quickly how to manage his lands, deal with political intrigue, and protect his people from bandits and rival lords. He is also a bit of a romantic, and his love interest could be a lady from a neighboring house, which adds another layer of drama to his story.
Maybe it's a bastard son or daughter of a powerful lord. In the world of Westeros, bastards often face discrimination, but this one could be different. They might have discovered some evidence or a loophole in the laws of inheritance that makes them a legitimate claimant. Their story could be about fighting against the stigma of being a bastard while trying to claim their rightful place.
Arya Stark could be a great main character in such a fanfiction. Imagine if she discovered she had latent sorcerer abilities. She could use her skills in disguise along with magic to infiltrate the most dangerous places. For example, she could magically change her appearance even more effectively than she does with her face - changing skills, and get closer to her enemies to take revenge.
Lelouch would be a great choice as the main character. His strategic mind and Geass power would make him a formidable force in the world of 'Game of Thrones'. He could easily navigate the political minefield of Westeros.
Maybe an outcast noble. They've been shunned by their family for their interest in the dark arts. So, they embrace necromancy fully and aim to use it to take revenge on those who wronged them. They start by raising the dead of their family's enemies, and gradually build an army of the undead. Their journey would be filled with self - discovery and moral quandaries as they delve deeper into the world of necromancy.