Percy could meet new characters that are hybrids of different mythological creatures. For instance, a centaur - mermaid hybrid. Also, he could encounter versions of his enemies who are actually his allies in this new universe. There could be a version of Luke who is a hero instead of a villain, and Percy would have to adjust his perception of people he thought he knew well.
He might meet alternate versions of his family members. Maybe a Percy from another universe whose mother is a powerful queen instead of a mortal.
He could meet some of the older campers who usually don't interact with the new arrivals right away. They might give him some special advice or warnings about the camp.
He might meet his former classmates who have changed a lot since he left. Maybe some new students who are intrigued by his reputation.
He could meet his old teachers again. They might be surprised to see him back after his long absence.
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.
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 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.
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 might meet a new teacher who is secretly a minor god or goddess in disguise.
Percy could meet the school bullies. And they might pick on him at first, but then be in for a big surprise when he shows his strength. Also, he might encounter some teachers who suspect there's something different about him. Maybe a science teacher who notices his quick reflexes during an experiment or a history teacher who is amazed by his knowledge of ancient mythology.
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.