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.
I don't have the exact name of the person who created the Omori manga at hand. It could be worth doing a search on manga databases or asking fellow manga enthusiasts for the answer.
Omori is purely fictional. The story and characters were imagined by the creators, not drawn from real-life events. It offers a fantastical and imaginative experience for players.
At the moment, there's no definite date for the Omori manga release. But you can follow the official social media accounts or manga forums for any potential news or leaks.
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.