Perhaps a corrupted demigod who has been influenced by the dark side. This demigod is jealous of Percy's fame and wants to prove himself superior. He steals the Staff of Hermes, thinking it will give him the power he needs to become the most powerful demigod in the world, and will stop at nothing to achieve his goal.
A powerful Titan who was once an ally of Hermes but now wants the staff for himself. He believes that with the staff, he can regain his former glory and overthrow the Olympians once and for all.
One idea could be that Percy discovers a hidden power within the Staff of Hermes that can open portals to different mythological realms not yet explored in the original series. He could then embark on a journey to these new places, facing new monsters and making unexpected alliances.
In some fanfictions, Hermes interacts with Percy as a mentor. He might offer advice on how to deal with other gods or mythical creatures. For example, he could tell Percy the weaknesses of a particular monster. In other stories, he may be more of an enigma, his actions and words being hard to decipher. Sometimes, Hermes and Percy form an alliance, like when they have to face a common threat together.
The theme of trickery also pops up a lot. Given Hermes' nature, there are often elements of trickery and deception in the fanfiction. This could be Hermes playing a trick on Percy or Percy using Hermes - like tactics to outwit his enemies. It makes for some exciting plot twists and keeps the readers on their toes.
There could also be a group of rogue demigods as antagonists. They might be jealous of Percy's adoption by Hermes and the powers that come with it. They could form a sort of anti - Percy alliance and try to sabotage his quests and make his life difficult among the demigod community.
It could be a powerful demigod who has been corrupted by power. This demigod wants to be the sole champion of the arena and will stop at nothing to defeat Percy.
In the fanfiction, it's possible that the boa staff is a reward for Percy helping out a magical creature. He could have saved a snake - like being that has a special bond with the boa staff. As a token of gratitude, the creature gives Percy the staff which then becomes an important part of his adventures.
A powerful demigod who has been corrupted by the idea of primordial power could be the main antagonist. They might believe that by allying with a primordial or obtaining their power, they can become greater than the gods. This demigod would then oppose Percy, who stands for the protection of the current order.
A powerful Amazonian sorceress who has turned against her own kind and is now trying to use dark magic to take over the world. She could see Percy as an obstacle in her path and constantly try to defeat him.
Maybe a rogue demigod who blames Percy for something and wants to make his exile permanent. This demigod could have some dark powers or be in league with some evil forces. They would constantly try to thwart Percy's attempts to return or find a way to clear his name.
Maybe an ancient sea demon that has possessed a pirate captain. This possessed captain is now leading his crew to cause chaos everywhere. They are attacking other pirate ships and even some demigod - related places. Percy has to find a way to exorcise the demon from the captain and stop the crew's rampage. The demon is very cunning and uses the pirate's knowledge of the sea to its advantage.