Well, perhaps Percy has been poisoned by a monster which is making him sick and his sneezes are a way of expelling the poison in a very strange way. His sneezes could start to change the environment around him, like turning the water in the camp's lake into a strange, fizzy substance. Grover could use his nature - sensing abilities to try and figure out what kind of poison it is, while Percy struggles with his increasing bouts of sneezing and the weakness that comes with his sickness.
One idea could be that Percy's sneeze accidentally triggers some sort of magical reaction. For example, it could cause a mini - tsunami in the middle of Camp Half - Blood because his powers are all jumbled up due to his sickness. Another plot could involve Percy sneezing and accidentally revealing his location to some monsters who are on the hunt for him while he's weakened by the illness. Maybe his friends have to figure out a way to keep him hidden and also cure his sickness at the same time.
His friends would first be really concerned. Annabeth would be in her problem - solving mode. She'd try to analyze the symptoms, not just the sneezing but also any other signs of Percy's sickness like his temperature or any strange marks on his body. Grover would stay by Percy's side, constantly offering moral support. He might also use his empathy link with nature to see if there are any plants that could help. Leo would likely try to invent some sort of device that could either stop the sneezing or figure out what's causing it, while Piper would use her charm - speak to try and soothe Percy when the sneezing fits get really bad.
One plot idea could be Percy having to team up with Hera to rescue a group of demigods who have been captured by an ancient monster. Hera, despite her usual aloofness, realizes she needs Percy's unique skills.
A plot could involve Percy's friends being in danger and the only way to save them is by making a deal with the entities in the Styx. He might have to sacrifice something precious, like a part of his powers, in order to gain their help. This would create a great internal conflict for Percy as he weighs the cost of saving his friends against his own abilities.
One idea could be that Bianca doesn't die. Instead, she discovers a hidden power within herself that can control time briefly. This power comes in handy when she and Percy are in a dangerous situation against a powerful monster. They are trapped in a maze - like lair, and Bianca uses her time - controlling power to freeze the monster, allowing them to escape.
One idea could be that Percy is reborn in a different era, like the Victorian age. He has to deal with new versions of the old monsters while also trying to figure out how his past life ties in. Maybe he discovers a long - lost prophecy about his rebirth that sets him on a new quest.
Another interesting plot could involve a vampire coven that has some sort of connection to the Greek gods. Maybe they want Percy for his unique demigod blood, which they believe has special powers. So Percy has to fight against this coven to keep his blood and his friends safe. This would bring in elements of both the Percy Jackson world and vampire lore. And it could lead to some really cool battles and alliances.
One idea could be a crossover where Harry and Percy meet during a magical event that blurs the boundaries between their worlds. Maybe a powerful dark force is causing chaos in both the wizarding world and the demigod world, and they have to team up to stop it. They could share their unique magical abilities and learn from each other's fighting styles.
A great plot could involve Percy being sent on a quest by a primordial. Since primordials are so powerful and ancient, the quest would likely be extremely dangerous and full of unknown challenges. For example, he might have to retrieve an ancient artifact that was lost in the time before the gods. This could lead to Percy facing not only physical challenges but also moral dilemmas as he deals with the primordial's motives.
A plot could involve Hera taking Percy on a journey through the past to show him the origins of the gods' power and how it relates to the current problems in the demigod world. This would give Percy a new perspective and knowledge that he can use to save the day. Or perhaps Percy accidentally stumbles upon a secret of Hera's that she's been hiding for centuries, and she has to enlist his help to keep it safe, all while other gods are trying to uncover it too.
One idea could be Percy gets cursed by Circe to forget his friends and mission, and he has to struggle to regain his memories while under Circe's influence. Another could be Circe teams up with Percy to fight a common enemy that threatens both the mortal and magical worlds. Or perhaps Percy has to rescue his friends from Circe's island after they are turned into animals, but he discovers that Circe has a more complex backstory and motives than he thought.