A plot where the gods of Olympus go on strike, leaving the demigods to deal with all the chaos on their own. The demigods would have to find a way to keep the world in order without the help of the gods, which would force them to rely on their own skills and relationships more than ever.
Maybe a story where Percy loses his memory and has to rediscover his identity while still being hunted by monsters. It could add a lot of mystery and tension as he tries to figure out who he is and why he's being targeted.
One idea could be exploring a new prophecy that involves Percy and his friends having to team up with some of the lesser - known gods. This would introduce new characters and powers into the story.