He could meet his younger self. This would be a really interesting encounter as he could either try to guide his younger self or be shocked at how different he was back then. Another possibility is meeting some of the ancient heroes that he only heard about before. For instance, he might come across Hercules and learn from him.
Percy could meet the early founders of Camp Half - Blood. He could witness how the camp was first established and perhaps offer his help in making it better from the start. He might also meet some of the gods in their younger forms. Zeus, for example, when he was still more impulsive and less in control. This would be a very different experience from dealing with the more 'mature' Zeus in the present time.
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 could meet his younger self, Anakin Skywalker. That would be a really interesting encounter. He might also meet Qui - Gon Jinn, who could sense the conflict within him. And there's a chance he'd cross paths with Yoda in his prime, which would lead to some intense confrontations or perhaps even unexpected alliances.
In such a fanfiction, Percy might use his knowledge of the future to change some key events in the past. For example, he could prevent some of the earlier disasters or conflicts in the Percy Jackson universe. Maybe he'd try to stop the first encounter with a certain monster in a different way to avoid the chaos it caused.
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.
It could be an ancient and powerful Titan who was thought to be defeated long ago. Since Percy is in the past, this Titan could be in his prime and see Percy as a threat to his plans of ruling over the world again. For example, Kronos might be even more powerful in this past era and Percy has to find a way to stop him without the help of his modern - day friends.
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.
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.
Maybe a corrupted past version of a demigod. For example, a demigod who was promised great power by the Titans and turned against the Olympians. This demigod could see Percy as an obstacle to their plans and try to stop him at every turn. They would know the time period well and could use that to their advantage against Percy.
One character could be a time - keeper or a wizard who has the power to send Percy back in time. This character could be an old and wise figure, full of knowledge about the past and the ways of time travel. Another could be a historical figure that Percy meets in the past. For example, a famous warrior or a philosopher who becomes Percy's ally or mentor.