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.
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.
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.
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.
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.
One common theme is loyalty. In the Roman Camp, loyalty to the cohort and the camp as a whole is highly emphasized, and fanfictions often explore characters' loyalty being tested. Another theme is identity. Demigods in the Roman Camp may struggle with their dual identity as half - god and half - mortal. Power struggles are also common, as different demigods may vie for leadership or recognition within the camp.
The plot could involve Percy being abandoned by camp because the camp leaders fear his growing power. He wanders the world, feeling lost and angry. Percy then encounters a mysterious figure who offers him a way to get revenge on the camp. However, Percy, being a hero at heart, realizes that taking revenge won't solve anything. Instead, he decides to find evidence to clear his name and expose the real danger that the camp is ignoring while he was being pushed away.