An interesting plot could be that Percy, as the son of Hecate, is shunned by some of the traditional demigods because of his different powers. He then has to prove himself not only to them but also to the magical community associated with Hecate. He could go on a journey to discover the true extent of his powers and find his place among both the demigod and magical worlds. This would involve him facing various challenges, making new friends, and maybe even finding a new way to combine his combat and magical skills.
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.