A maester who discovers the existence of the giantess could be the main character. He would be intrigued by her and study her. His knowledge and curiosity could lead to him being the one to bridge the gap between the giantess and the rest of Westeros. He could write about her in his chronicles, and his actions would determine whether the giantess is seen as a friend or foe to the kingdoms.
Arya Stark could potentially be a futa character in a different take on the story. Given her already established skills in disguise, fighting, and her thirst for revenge, as a futa, she could have additional physical and strategic advantages. She could use her new - found abilities to further her goals of avenging her family and bringing justice to those who wronged them. This would transform her character arc in a very interesting way.
One possible plot could be that a giantess emerges from the far North, beyond the Wall. She could be a remnant of an ancient race. The characters in the Game of Thrones world might initially see her as a threat. For example, the Night's Watch could fear she is allied with the White Walkers. But as the story unfolds, it turns out she has her own motives. Maybe she is seeking a lost relic that was stolen from her kind long ago. And she might form an unexpected alliance with a group of outcasts in Westeros to achieve her goal.
The main character could be a popular girl in school who becomes a giantess overnight. This completely changes her social status. Some of her so - called friends turn their backs on her, but she makes new ones among the outcasts. She uses her new size to stand up to the school bullies and in the process, she transforms the school into a more inclusive and accepting place.
It could be Daenerys Targaryen. She's already a strong and unique character. If she were to be the main character in such a story, the breast expansion could be part of a new magical transformation she undergoes as she continues her journey to claim the Iron Throne. It could add an interesting layer to her character, as she already has the power of her dragons and this new physical change might make her even more of a figure to be reckoned with in the eyes of her followers and enemies alike.
Perhaps the main character could be a female machinist. In a world dominated by men, she has to fight against prejudice while also using her skills. She could be from one of the noble houses but has a secret love for machinery. She might be working on a project to build a flying machine, which is seen as crazy by others, but she is determined to prove them wrong and change the face of warfare in Westeros.
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.
A lost princess from Westeros who washes ashore in Ulthos. She quickly realizes that Ulthos is a land full of secrets and danger. With her charm and the political knowledge she learned in Westeros, she tries to navigate the complex power structures in Ulthos and make a name for herself.
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.
A demon who has taken on human form could be the main character. He could be disguised as a knight or a noble, secretly using his demonic powers to influence the events in the Seven Kingdoms.