One possible disease in fanfiction could be a 'god - touch' disease. Since he has so much interaction with the gods, perhaps there's a negative side effect. It could be something that makes his body slowly turn to stone, like Medusa's victims, but in a more gradual and complex way. This would force him to seek out powerful cures and make for a great story.
Perhaps the Sacred Gear has a mind of its own. It could communicate with Percy in his dreams or during times of great stress. The Sacred Gear might have its own goals and motives that don't always align with Percy's. This internal conflict, along with external threats, would make for a very interesting and complex fanfiction.
One possible plot could be that the entire Greek pantheon is thrown into chaos. Percy might be seen as a huge threat, and other gods and goddesses would either try to hunt him down or form alliances with him out of fear or a desire for power. Demigods would be forced to take sides, leading to massive battles among them.
One possible disease could be a magical malady like a spell - induced curse that slowly drains her magic. Another might be a fictional wizarding disease that affects her ability to perform spells accurately. For example, a 'Spell - Fade' disease where her incantations start to lose their potency over time.
In a more complex plot, Percy could be forced into a boxing match by a powerful enemy who wants to humiliate him. But Percy turns it into an opportunity to unite the demigods. He uses the boxing ring as a platform to give inspiring speeches between rounds. And as the matches progress, he gradually gains the respect of both his allies and former foes through his tenacity and skill in the ring.
Well, in fanfiction, one reason could be the loss of his friends. Percy has been through so many battles, and if he loses someone close like Annabeth or Grover, it could send him into a deep depression. Another reason might be the burden of his prophecies. He's constantly expected to save the world, and that kind of pressure can wear a person down over time.
Perhaps Percy has proven his worth through his unwavering loyalty and courage. Loki, who is used to seeing self - serving and deceitful behavior among the gods and mortals, is impressed by Percy's pure - heartedness. His actions of always protecting his friends and standing up for what is right might make Loki feel that he is worthy of respect, leading to Loki bowing to him.
Maybe Percy unknowingly breaks some ancient and very strict laws of the Gods. For example, he might interfere with some sacred rituals or places that are off - limits to mortals, even demigods like him. In the fanfiction world, the Gods could be very unforgiving when it comes to such violations and decide to kill him as a result.
One possible reason could be that Percy has shown some extraordinary power that even Thor, a powerful god, respects. Maybe Percy has defeated a common enemy in a really remarkable way that makes Thor recognize his strength and kneel in admiration.
One possible plot could be Percy waking up one day and finding himself back at the start of his journey at Camp Half - Blood, but with all his memories intact. He could then use this knowledge to avoid the mistakes he made before, like getting in unnecessary fights with some of the other campers. He might also try to befriend some characters earlier who he didn't get to know well in the original story.
One possible plotline could be Percy trying to escape the Hydra experiment facility while using his demigod powers. He might have to fight off Hydra agents and other dangerous creatures they've created through their experiments.