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.
I'm fairly certain there is. Fanfiction writers often like to cross - over different fictional universes, and Percy Jackson and Excalibur are both rich sources of inspiration. Writers could explore how Percy, with his demigod abilities, might interact with the power and mystery of Excalibur. There could be all kinds of plotlines, from Percy finding Excalibur on a quest to it being a key element in a battle against a new, powerful enemy.
Yes, it is quite popular. Many fans enjoy exploring different relationship dynamics for Percy Jackson in fanfiction, and the 'harem' concept, where he has multiple romantic interests, offers a lot of creative possibilities for writers to play with different character interactions and storylines.
Well, let's say Percy has a disease that affects his powers. His relationship with his demigod friends could change. They might start to see him as a liability instead of a strong leader. And his relationship with the gods could also be altered. The gods might either try to use his weakened state for their own ends or completely abandon him, which would have a huge impact on how he views them and his place in the world.
I'm not entirely sure as it's a very specific fanfiction concept. But it might involve Percy Jackson, a character from the Percy Jackson series, somehow obtaining the Rinnegan, which is from the Naruto universe. It could be a really wild crossover story with Percy using the powers of the Rinnegan in his own adventures.
The plot could involve Percy waking up one day and finding a tail instead of his legs. At first, he's confused and a bit scared. But then he realizes it gives him some new abilities, like super - fast swimming. He could then go on a quest where his tail is both an advantage and a hindrance. For example, it might be hard to move on land, but in water, he can out - swim any monster.
There's also 'Female Percy: A New Heroine'. Here, Female Percy brings a fresh perspective to the story. Her fighting style is different, more fluid perhaps. And her relationships with the gods are also unique. Instead of the typical male bravado, she uses her wits and charm to get through tough situations.