Daenerys Targaryen could also be a main character in an emperor - like role. She already had a strong claim to the Iron Throne and was a powerful leader. In a fanfiction, she could fully embrace her role as an all - powerful emperor, perhaps even uniting the Seven Kingdoms under her rule in a more forceful way than in the original series.
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.
The Mother. In the existing lore, she is a major figure. In fanfiction, she could be more actively involved in the events of Westeros. She could be a guiding force, using her powers to heal the sick and wounded, and trying to prevent the bloodshed that is so common in the Game of Thrones world.
The Hound could be a Futa character. His already rough and tough exterior would be combined with a different physicality. His journey of redemption or his loyalty to certain characters could be explored in a new light, and his interactions with other fighters in the story would be more complex.
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.
Some of the more independent lords or ladies in Westeros could be allies. For instance, Lady Brienne of Tarth. She is a paragon of honor and if the bastard displays honorable intentions, she could offer her sword and protection. Also, the wildlings might see the bastard as a way to gain some influence in the south. If the bastard offers them something in return, like a place to settle or resources, they could form an alliance.
There's also a possibility of her allying with some of the more neutral houses. Houses that were tired of the constant fighting between the big players. Her daughter could offer them stability and protection in exchange for their loyalty. She could present herself as a new hope for a more peaceful Westeros, which might attract these houses to her side.
In some more creative fanfictions, an outsider, like a mysterious traveler from the Free Cities, could be her love interest. He could bring new ideas and perspectives to the North, and their relationship could be a way to bridge the gap between the North and the rest of the world.