In fanfiction, when Percy Jackson goes to highschool, he may encounter a group of students who are descendants of minor mythological creatures. They might not be as powerful as demigods but still have some unique abilities. He could also meet a school counselor who has a special connection to the world of the gods and tries to help Percy balance his two lives. Additionally, there could be a transfer student who is actually a demigod from another camp and they form an instant bond.
He might meet a new teacher who is secretly a minor god or goddess in disguise.
If Percy goes to highschool in fanfiction, he could meet other students with latent demigod powers that they're not even aware of yet. Or perhaps he'll come across a mortal who has an uncanny knowledge of Greek mythology and is intrigued by Percy.
He could meet some Aboriginal heroes or spirits. They might have their own powers and knowledge that Percy can learn from.
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.
In fanfiction, if Percy Jackson goes to highschool, he might face challenges like hiding his demigod powers from his classmates. He could also encounter new mythical creatures that are drawn to the school environment.
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.
If Percy Jackson goes to Valhalla in fanfiction, he could meet a whole bunch of Norse heroes. There's Sigurd, the dragon - slayer. And then there are the Valkyries who choose who gets to go to Valhalla. He might also run into Loki, who is always causing some sort of trouble. Percy would be in for quite an adventure as he tries to figure out who he can trust among these new and often unpredictable characters.
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.
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.
He might first meet a human. Azeroth has a lot of human settlements, and it's likely that Percy would stumble upon a human village or town first. The humans would be both curious and cautious about him, especially if they notice his strange 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.
There's also a chance that he'd meet Lydia first. Lydia has her banshee powers, and she might be drawn to Percy because of his otherworldly presence. She could sense something about him that's not of this world, and approach him out of curiosity. Their meeting could be a very intense and mysterious one, with Lydia trying to figure out what exactly Percy is.