One interesting headcanon could be about the backstories of the Servants. For example, there might be a headcanon that a certain Servant had a secret friendship in their past life that influenced their character in the game. Another could be related to the magic system in the Fate Grand Order universe, like a unique way of casting spells that isn't shown in the main story.
Well, a popular headcanon is that Percy can see the memories of the ocean. So in fanfiction, he might touch an old piece of coral and suddenly be flooded with images of ancient sea battles or long - lost civilizations. Also, some think that Annabeth has a fear of heights that she hides really well because she doesn't want to seem weak. As for Grover, perhaps he has a family of satyrs in a hidden grove that he secretly visits and they tell him old satyr tales that he then uses to his advantage on quests.
I like the headcanon where Annabeth is a master of disguise. In fanfiction, she can blend into any crowd, not just because of her intelligence but also because she has some magical items that help her change her appearance. It makes her even more resourceful on quests. Grover, on the other hand, has a headcanon where he can grow plants with just a touch. So if they're in a barren area, he can create a little oasis for them to rest in. This shows his connection to nature in a more powerful way.
One idea could be that Neville Longbottom is secretly a powerful wizard all along but has been suppressing his power due to self - doubt. Another could be that the Hogwarts portraits have their own little society and gossip network within the castle walls. And a third one might be that Hagrid has a secret family of magical creatures in the Forbidden Forest that he takes care of.
For demigods, it could either reinforce their existing views or introduce new perspectives. If a fanfiction delves deeper into a certain aspect of their world like the powers of a god, they might gain a new understanding. Mortals would have a more comprehensive view of the PJO world. They would learn about different characters and their backstories that might not be fully explored in the original books.
It can add more depth to minor characters. For example, by creating headcanons about characters like Lavender Brown, we get to see a side of the Hogwarts student body that wasn't fully explored in the original. It gives them more personality and backstory.
It can be considered common. There are a large number of PJO fans out there, and fanfiction is a popular medium in fan communities. Since the official books have a set storyline, fanfiction offers a chance to expand on it, so many 'mortals' (fans) are attracted to it.
One great 'athena pjo fanfic' is 'Athena's Wisdom'. It really delves into Athena's character in the Percy Jackson universe. The author does a wonderful job of showing her wisdom and how she interacts with the other characters. It has some really interesting plot twists that keep you hooked from start to finish.
I'm not entirely sure specifically as it could refer to different things, but generally a 'graphic novel' is a book in comic format. If 'pjo' stands for 'Percy Jackson and the Olympians', then it would be about the adventures of Percy Jackson in a graphic novel format. It might include his encounters with Greek gods, monsters, and his quests.
Well, one reason people read PJO fanfic is that they want more of the Percy Jackson world. The official books can't cover every single idea or relationship. Fanfiction fills those gaps. It might also be that some readers enjoy the different writing styles of fanfic authors. They can find unique interpretations of the characters and settings, which adds to the allure of the PJO universe.