A common theme is the clash of different worlds. Percy, coming from the demigod world, and Hecate, from the world of magic, their relationship often involves dealing with the differences between their two realms.
There's also 'Hecate's Heart and Percy' which delves deep into the concept of their love. It starts with Percy stumbling upon Hecate's realm and from there, a slow - burning romance develops. The story is filled with emotional moments and interesting plot twists that keep you hooked.
Another theme is the idea of forbidden love. In the context of Greek mythology, a relationship between a demigod and a goddess can be seen as improper or against the rules. Many fanfictions play with this idea, showing how Percy and Hecate have to overcome obstacles related to this.
The plot could also be about the daughter of Hecate uncovering a dark magic plot that endangers Camp Half - Blood. She initially tries to handle it on her own but realizes she needs help. Percy Jackson gets involved and they start to investigate. Along the way, they discover that the source of the problem is an ancient curse related to Hecate herself. The daughter has to come to terms with her heritage and use her powers in ways she never thought possible to break the curse and save the camp.
The Titans could still be antagonists. Since Percy has a connection to Hecate, they might see him as a threat to their plans of overthrowing the gods. They could try to capture him to use his powers for their own evil purposes.
Obviously, the daughter of Hecate would be a main character. Percy Jackson himself would also be a key character. And maybe Annabeth could be involved too. She could be a bit skeptical of the daughter of Hecate at first because of her association with magic, which is often seen as unpredictable in the Percy Jackson world.
The Titans could still be antagonists. They might see Percy as a threat in his new role as the champion of Hecate. Another could be some rogue demigods who are jealous of Percy's new power and want it for themselves. And there could be magical creatures that have been corrupted by dark magic and oppose Percy's efforts to bring balance with Hecate's magic.
You could also develop his character by showing his interactions with other gods and goddesses. As the son of Hecate, he might attract the attention of some deities who either want to help or hinder him. His reactions to their offers and threats would shape his character. For example, if a god offers him more power but at a great cost, how does Percy respond? Does he accept the power out of desperation or does he hold true to his values? This would show his growth and development.
Well, a plot could involve Percy getting caught in a magical feud between different factions related to Hecate. He might have to use his combat skills along with his newfound magical abilities to resolve the situation. Also, a story where Percy has to protect a sacred place related to Hecate from monsters would be interesting. He could learn about the secrets of Hecate's magic in the process.
One popular Percy and Hecate love fanfiction is 'Percy and Hecate's Unlikely Bond' on FanFiction.net. It explores how their different powers and personalities draw them together in a world filled with mythological challenges.