Athena would be a likely candidate. Given Percy's growth in intelligence and strategy over his adventures, Athena might consider him a worthy addition to her circle. Demeter could also be in the harem. Percy's connection to the earth and his ability to grow and adapt could attract her. And of course, Poseidon's relationship with Percy could lead to some interesting dynamics if Amphitrite, Poseidon's wife, were to be part of the harem in some way.
Tsukuyomi could be one. In Japanese mythology, she is a moon goddess. Her calm and mysterious nature would add an interesting dynamic to the harem. She could use her lunar powers to help Naruto in battles, like creating illusions based on moonlight.
The other gods who oppose the relationship between Percy and the goddess could be antagonists. They might see it as a disruption of the natural order or a threat to their power. For example, if the goddess is from a certain pantheon and her pregnancy by a mortal like Percy goes against their rules, those traditionalist gods would be against it.
Annabeth could be a likely member as she has a deep connection with Percy already. Another could be Piper, given her charm - speaking abilities and her friendship with Percy. And maybe Hazel, with her unique powers and kind heart.
Annabeth could be one. She's been Percy's close friend and has a strong connection with him. Also, Piper could be a part as she has her own charm and abilities. And maybe Thalia, with her fierce nature.
It could be Aphrodite. Since she is the goddess of love, it would be an interesting twist. Percy might initially resist her charm but eventually be won over by her true self. Their marriage could bring a new understanding of love to the world of the fanfiction.
Obviously, Annabeth Chase could be one of the main characters in the harem. She already has a strong connection with Percy. Then, there could be Thalia Grace, with her cool and powerful demigod nature. From other mythologies, Artemis could be an interesting addition. Despite her vow of chastity, in a fanfiction world, she could be drawn to Percy in a different way. Also, maybe a character like Katara from Avatar: The Last Airbender could be brought in through some sort of crossover magic, adding an element of water - bending to the mix.
It could be Annabeth, who was made a goddess in some interpretations. Their kids would be really smart and brave, inheriting Percy's heroism and Annabeth's wisdom.
Another possibility is Hera. Although not typically described as 'curvy' in the original myths in that way, in a fanfiction, she could be reimagined. Her role as queen of the gods could bring a whole new dynamic to the story if she's the curvy goddess in this fanfiction. She might be involved in some sort of power struggle or family drama that Percy gets dragged into.
One possibility could be Morgana. In some interpretations, she has powerful magic and could be reimagined as a more benevolent Goddess of Magic in the Harry Potter universe. Another could be a completely new character created specifically for the fanfiction, perhaps an ancient witch who has been watching over the wizarding world since its inception.
One could be Demeter. Since she is the goddess of the harvest, her relationship with Percy could involve themes of growth and renewal. And then there's Persephone. Her complex relationship with the underworld and the seasons could add a dark and mysterious element to a fanfiction where she is Percy's soulmate. There are also less common choices like Nike, the goddess of victory. If she were Percy's soulmate, it could focus on his success in battles and quests.