One possible plot could be the son growing up in the shadow of his mother's power-hungry ways. He might struggle with the expectations placed on him due to his family name. For example, he could be groomed to be a political pawn but secretly desires a more peaceful life away from the power struggles of King's Landing.
One possible plot could be that their son discovers the truth about his parents' tumultuous relationship. He might struggle with the knowledge that his mother had affairs and his father was not the ideal man he seemed. This could lead him on a journey of self - discovery, trying to figure out his place in the Seven Kingdoms.
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.
He could influence events by being a voice of reason in Cersei's court. If he's intelligent and perceptive, he might be able to persuade his mother to make more diplomatic decisions rather than always resorting to force.
One possibility could be that the oc, being the son of Drogo, has a claim to the Dothraki leadership. He could strive to unite the Dothraki clans in a different way than Daenerys did. Maybe he focuses on forging alliances with the other kingdoms based on mutual trade and protection rather than conquest.
One idea could be that the OC is a healer from a far - off land who comes to King's Landing. Cersei initially distrusts the OC but as time passes, she realizes the OC's skills are invaluable, especially when it comes to treating her children's ailments. This could lead to a complex relationship where Cersei struggles between her pride and her need for the OC.
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.
One possible storyline could be that Cersei's daughter rebels against her mother's tyrannical ways. She might see the destruction Cersei has caused and decides to take a different path, perhaps allying with the Starks to bring peace to Westeros.
One possible plotline could be Cersei's son growing up to be a more benevolent ruler than his mother. He could see the destruction she has caused and try to make amends. Maybe he forms alliances with former enemies to bring peace to the Seven Kingdoms.
In an interesting fanfiction plot, Cersei's son could be exiled from King's Landing. During his exile, he forms a band of outcasts. Together, they build a new community away from the power struggles of the capital. As he grows, he realizes that his mother's way of ruling is wrong. Eventually, he returns to King's Landing with his new - found wisdom and tries to overthrow his mother's tyrannical rule, not to take over but to establish a more democratic and fair system for the people of Westeros.
In a 'Game of Thrones Epilogue Fanfiction', it could be about the adventures of the next generation. The children of the main characters might discover new threats or mysteries. For instance, they could find hidden secrets about the White Walkers that were not fully explored in the original series, leading to a new quest.