Well, he might face new monsters. For example, some ancient and more powerful versions of the ones he's already fought. Maybe there are new quests waiting for him at the camp, like finding a lost artifact that has the power to either save or destroy the camp.
Another possibility is that there are internal camp politics or mysteries that Percy gets involved in upon his return. There could be a group within the camp that has some secret plans, and Percy, being the hero he is, has to figure out what's going on and stop any potential threats to the harmony of the camp.
Well, in many fanfictions, Percy Jackson's class could get involved in a quest at the camp. Maybe they have to retrieve a lost artifact or solve a mystery that's been plaguing the camp. They could also form new friendships and rivalries with the other campers. There might be some intense training sessions where they learn to use their newfound powers. And who knows, they could even encounter some long - lost gods or goddesses who have a special task for them.
There could be a new camper who has some really unique powers. Maybe a child of a minor god that we haven't heard much about in the original series. This new character could be both a friend and a rival to Percy.
Well, he might disappear because of a powerful curse from an ancient enemy. Maybe he gets trapped in a different dimension. When he returns, he could have new powers or knowledge. For example, he might have learned some secret magic from the other dimension that helps him defeat a new threat in the Percy Jackson world.
There could be new challenges waiting for him at the camp. Maybe a new prophecy that involves him and he has to quickly get back into the action of training and preparing. He might also find some changes in the camp, like new cabins or new campers who look up to him.
One possible plotline could be Percy having nightmares about his time in Tartarus. He might struggle with PTSD, which could affect his relationships with his friends and family at Camp Half - Blood. Another could be that he brings back some sort of dark power or knowledge from Tartarus that he has to figure out how to control or use for good. Maybe he discovers a way to close the entrance to Tartarus permanently so that no more monsters can come out and endanger the demigod world.
One reason could be if he was wrongly accused of stealing something important from the camp. Maybe a powerful magical item goes missing and Percy is the prime suspect because of his past issues with authority. Another reason might be if he accidentally endangers the camp in some way during a mission or training exercise. For example, if he unknowingly leads some dangerous monsters back to the camp.
He could go on a quest to find a long - lost artifact that was stolen from the camp. Maybe it's a powerful weapon that once belonged to one of the gods. With his leadership skills and the help of his friends, he would face many challenges along the way.
Percy as a pirate captain would likely have a ship with special features. It could be enchanted in some way, perhaps having the ability to sail faster than normal ships due to some demigod - related magic. His cabin on the ship might be filled with mementos from his previous adventures on land, like a trident he once found or a piece of armor from a defeated monster.
In some fanfictions, the Nemean Lion might be given more human - like intelligence. So, Percy has to outwit it rather than just relying on his physical strength. He could use his knowledge of Greek mythology to find the lion's weakness that others don't know about.
He might go on a solo adventure to discover more about his powers. He could encounter new mythical creatures that he hadn't seen around the camp. Maybe he'll meet some other demigods who are on the run or have their own secret quests.