In this fanfiction, it's possible that Percy undergoes a significant change in his personality. The trials and terrors of Tartarus could make him more hardened, or it could lead to a new sense of compassion within him. For example, he might see the plight of some of the monsters in Tartarus and have a different view on them, which is a very interesting development.
Annabeth might also be a main character. She could be either with Percy in Tartarus or be a driving force for him from outside, like trying to find a way to rescue him or communicate with him in some way.
One common change is that he's more cautious. He's seen how dangerous the world can be in Tartarus, so he doesn't rush into things as easily. Another is that he might be more empathetic towards others who have been through trauma, since he now has his own to deal with.
Perhaps a plot where Percy forms an unlikely alliance with some of the more neutral or even hostile creatures in Tartarus. He could use his charm and wits to convince them to help him in his struggle against the greater evils of that place. In this way, he learns more about the complex hierarchy and politics within Tartarus.
One interesting idea could be Percy's struggle to survive in Tartarus. He might have to face new and terrifying monsters there. Another idea could be his journey of self - discovery in such a harsh environment. Maybe he'll find new powers or a deeper understanding of his own capabilities.
Some Tartarus fanfictions focus on Percy's mental state while in Tartarus. It could explore his nightmares and how he copes with the overwhelming sense of despair. For instance, he might have visions of his friends and family in danger back on the surface, and he has to find the strength within himself to keep going in the face of such mental torture.
One interesting scenario could be Percy and Annabeth finding a hidden, powerful weapon in Tartarus that gives them an edge against the monsters there. Maybe it's a sword that can cut through the very fabric of Tartarus's darkness.
Maybe Percy discovers a long - lost power within himself in Tartarus. This power could be related to his connection with the Underworld. He could use it to communicate with the souls trapped there, and gain knowledge that helps him rule Tartarus. He could also start training with some of the toughest monsters to become even stronger and assert his rule.
Perhaps there's a plotline where Percy discovers new and dark powers within Tartarus that he has to learn to control. This could lead to him having to make difficult choices about whether to use these powers for good or risk being consumed by them. He may also encounter other fallen or corrupted beings that either try to help or hinder him on his journey.
One interesting plot could be Percy's struggle to control his new powers as the God of Tartarus. He might face challenges from other gods who are threatened by his rise. Another could be his journey to reform Tartarus, making it less of a hellish place. For example, he could try to bring some order to the chaotic entities that reside there.
One interesting plot could be Percy's struggle to survive in Tartarus alone. He might have to face the various monsters there without his usual allies. Another plot could involve him finding a way to escape Tartarus using his wits and the powers he has. Maybe he discovers some secrets about Tartarus that no one else knows.