A merchant who has amassed a great fortune could become a new lord. He uses his wealth to buy a lordship when the previous lord falls into debt. He is shrewd and business - minded. He brings new economic ideas to his lands, like building better trade routes and introducing new industries. However, the traditional nobles look down on him because of his commoner background, and he has to constantly prove himself worthy of his new title.
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 former soldier who has shown great courage during a battle. He could have been the one to hold the line at Moat Cailin during a crucial fight, and in recognition of his valor, he's made the new lord. His military experience would be a great asset in ruling the area.
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.
One idea could be a new lord who is a long - lost heir to a powerful but fallen house. He starts from nothing, gathering a small group of loyal followers, some outcasts and misfits. He uses his wits and new - found knowledge of forgotten battle tactics to slowly rise in power. He might form an alliance with a group of local peasants who are tired of being oppressed by the existing lords. Through their combined efforts, they start to build a new, more just community which catches the attention of the greater powers in Westeros.
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.
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.