There's a possibility that it's a new character introduced in the fanfiction. This new character could be someone who has a connection to the events of the giant war, like a relative of a fallen hero. They might be exploring the post - war world, uncovering secrets that were hidden during the war, and their journey could be the main focus of the story.
Another option could be a new demigod. A character who was not very involved in the war but is now thrust into the post - war chaos. This new character could bring a fresh perspective and could be learning about the demigod world and its history while also trying to help in the rebuilding and healing process.
Maybe the characters start to rebuild their lives. For example, they might focus on restoring the damaged places and re - establishing relationships that were strained during the war.
It could be some remaining monsters that survived the giant war and are now seeking revenge. They might target Nico specifically because of his role in the war. Another possibility is some rogue demigods who didn't like the outcome of the war and blame Nico for it.
Nico di Angelo could be a main character as well. His powers over the dead and his unique perspective could be valuable. He might be dealing with the influx of new ghosts due to the large number of casualties in the war and trying to find a way to give them peace.
It could be a rogue demigod who was on the losing side of the Giant War and is now seeking revenge. This demigod might have found some dark power or be allied with some minor but dangerous mythical creatures.
Perhaps some of the demigods from other camps that he didn't interact much with before could become his new allies. They might see his leadership during the war and want to be part of his group, to learn from him and fight alongside him in any future conflicts.
I think Nico di Angelo could also be a central character. He has his own unique powers and backstory, and after the giant war, he might be involved in some of the darker or more mysterious aspects of the post - war world, like dealing with any remnants of the Underworld's connection to the giants' plans.
There could be new demigods. Maybe some who were hidden during the giant war and now come out. For example, a demigod of a less - known god or goddess.
One possible plot could be Percy and his friends dealing with the aftermath of the war in the mortal world. Maybe they have to hide their demigod identities even more carefully as the war has drawn some unwanted attention. They could also be working on rebuilding Camp Half - Blood and trying to make it safer for new demigods.
A rogue ninja from a formerly warring village could be the main antagonist. This ninja might hold a grudge against the new peace and wants to disrupt it. He could be powerful and have his own group of followers who are also discontent with the current situation.