Percy may also meet some of the Titans who were overthrown by the Olympians. For example, Prometheus, who was known for stealing fire from the gods. Prometheus could share some ancient knowledge with Percy that is crucial for his journey in ancient Greece. And there's always the possibility of meeting the Muses, who could inspire Percy in different ways, perhaps by giving him the gift of better storytelling or inspiring him to create a new strategy to deal with his enemies.
In many fanfictions, Percy is likely to meet ancient Greek heroes such as Heracles. They could engage in some friendly competitions or Heracles could offer his strength to help Percy in a difficult situation. Another character he might come across is Pandora. She could be either a help or a hindrance depending on the plot of the fanfiction as she is associated with the box that contained all the evils of the world.
There's a good chance he'll meet the goddess Athena. Since Percy has a connection to the world of the gods, and Athena is known for her wisdom and strategic thinking, she could offer Percy some valuable advice. Maybe she'd help him navigate the complex social and political situations in ancient Greece or teach him some new ways to use his demigod powers.
He could meet other demigods from different pantheons. Since it's a different world, there might be demigods associated with gods that Percy has never heard of before.
He might meet a new teacher who is secretly a minor god or goddess in disguise.
Percy might meet an old enemy who has somehow been resurrected or changed in the future. This enemy could be more powerful and have a new plan to take over the world or destroy the demigods. Or he could meet a group of time - traveling demigods from an even further future who are trying to fix something in Percy's future that has gone wrong.
He might meet Luke before he turned bad. Percy could try to change Luke's fate, convince him not to follow Kronos. It would be a great chance to save a friend and prevent a lot of the chaos that happened later.
Well, often in these fanfictions, Percy gets to interact with the ancient Greek heroes in a more in - depth way. He could team up with Achilles and learn some new combat skills from him. And he might be involved in some sort of mystery that is related to the lost artifacts of ancient Greece. Maybe he has to find a certain magical item that was once used by Zeus himself to save the world from a new threat.
Percy might run into Loki. Loki is known for his trickery. He could either befriend Percy for his own devious plans or try to deceive him. Percy, being used to dealing with tricky situations in his own world, would have to be on his guard against Loki's schemes.
He could also meet some minor nymphs or spirits that inhabit the island. These beings might be curious about Percy's return and have their own stories to tell. For instance, there could be a water nymph who has some knowledge about a prophecy related to Percy and his return to Ogygia. Or a spirit that has been observing the changes in Ogygia since Percy last left and can give him some insights on what has been going on in his absence.
He might meet some talented witches and wizards. Maybe there would be a student who is an expert in magical beasts and they could share stories about their encounters with strange creatures.
He would probably meet Eragon. Eragon is a key figure in Alagaesia, and their encounter would be full of surprises as they come from different worlds with different powers.