He could meet an old wizard who has been guarding the vault for centuries. This wizard could either be a friend or a foe, depending on Percy's actions.
Percy might encounter a group of thieves who are also after the contents of the vault. They could team up with him or try to double - cross him. There's also a possibility that he meets a spirit or a ghost within the vault that has some connection to the items inside.
In the fanfiction, Percy could come across a lost relative who was trapped in the vault. This relative could have important information about Percy's family history and the significance of the vault's contents. Or he could meet a magical being that is the key to unlocking all the secrets within the vault.
Well, in this fanfiction, Percy might discover all sorts of treasures in the vault. Maybe it's filled with ancient magical artifacts that could give him new powers or abilities. It could also be a place full of secrets about his past or the world he lives in.
He could meet his future self. This encounter might be full of surprises as his future self could have different views and experiences. His future self might be more jaded or more powerful.
There's a chance that Percy meets the personification of Tartarus itself. It could be a shadowy, all - powerful being that tries to break Percy's spirit. Percy, being the brave hero he is, would have to resist and find a way to either outwit or overpower this entity in order to continue his journey in Tartarus.
Perhaps Percy will meet a group of environmentalists. Given San Francisco's reputation for being environmentally conscious, they could be fighting against some sort of sea - related pollution that Percy gets involved in. He might use his powers and knowledge to assist them, and in the process, make some good friends.
Percy might run into his old friends' ancestors. This could create some really strange and exciting interactions. For instance, if he meets an ancestor of Annabeth, he could learn things about her family history that she doesn't even know.
He might meet some native Alaskan spirits. These spirits could be like the nature spirits he has encountered before but with their own unique Alaskan characteristics.
There could be a school janitor who is actually an old demigod who retired from the hero business. He could give Percy some valuable advice based on his own experiences. And of course, Percy will surely meet Annabeth's classmates, some of whom could be friendly and some not so much, depending on how they react to the new boy in school.
He could meet some Aboriginal heroes or spirits. They might have their own powers and knowledge that Percy can learn from.
Percy might encounter Baba Yaga. She is a well - known figure in Russian folklore. She could be an adversary, testing Percy's skills and his knowledge of the magical world. Or perhaps she could be a source of information if Percy manages to earn her respect. He could also meet the Rusalki, water nymphs from Russian mythology. Given Percy's connection to water, it would be interesting to see how he interacts with them.
The child could be a prophesied one. In the world of Percy Jackson, prophecies are common. So, this child could be the one foretold in some ancient prophecy that is linked to Tartarus. Percy finding the child sets off a chain of events that will either save or doom the world. Maybe the child is the key to closing a portal in Tartarus that is releasing evil into the mortal world, and Percy is the only one who can help the child fulfill this great task.