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.
Well, there would obviously be the bastards themselves. They could be given names like Joffrey Jr. (even if it's just a fan - given name), a son who has all of Cersei's cunning and a thirst for power. Then there could be a daughter named Myrcella - like, after the real Myrcella but with a different personality. She might be more fierce and independent, not willing to be a pawn in the political games like the original Myrcella.
One idea could be that Cersei's bastard rises to power in secret. He or she could form alliances with unlikely characters, like some of the smallfolk who have been overlooked. They might use their wits and charm to gain support, all while hiding their true identity from the powerful families in Westeros.
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.
Sansa Stark could be an interesting pairing. It could be a story of two strong - willed women from opposing sides. They could either become bitter enemies or, in a more creative take, find common ground and form an unexpected alliance. Also, Margaery Tyrell is a great character to pair with Cersei. Their power struggle in the show was fascinating, and fanfiction could take it in new directions, like Margaery actually befriending Cersei and showing her a different way to rule.
Even characters from other villages could potentially be allies. For example, Gaara. In the game world, he and Naruto could form a powerful duo, with Gaara's sand - based attacks complementing Naruto's Rasengan - like moves. They could fight together against common in - game threats.
An OC who is a skilled assassin would be a good choice. They are hired to kill someone in King's Landing but cross paths with Cersei. Cersei, instead of having the OC killed, decides to use their skills for her own purposes. The assassin has their own code and ethics, which would constantly be at odds with Cersei's methods.
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.
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.
Perhaps in the fanfiction, Cersei's son discovers a hidden power within himself. He uses this power not for domination like his mother would, but to heal the broken kingdom. He might travel across the land, learning from the common people and using his new - found abilities to bring prosperity.