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.
Another plot might involve Percy's internal struggle. Staying in Tartarus for a long time could make him question his own identity and purpose. He might start to doubt his heroism as he is constantly surrounded by darkness and despair. He has to fight not only the external threats but also the demons within himself, which could be a really deep and engaging plot.
One possible consequence could be that he becomes even more powerful in a dark way. Tartarus is full of dangerous and ancient forces. Staying there might expose him to unique and powerful magics that change him. For example, he could gain new combat skills or the ability to control certain Tartarus - born entities.
Another character that could be involved is Nico di Angelo. Since he has some experience with the Underworld and Tartarus, he might try to find a way to communicate with Percy or even attempt a rescue mission. His powers and knowledge would play an important role in any interaction with Percy in Tartarus.
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.
One more plot idea could be that Percy and Annabeth are somehow affected by the dark magic of Tartarus. It changes them in some way, either physically or mentally. They then have to deal with these changes while still trying to escape. This could lead to some really intense internal and external conflicts as they struggle to stay true to themselves and also find a way out of Tartarus.
It's also possible that there could be a new, unknown force in Tartarus that has been awakened by Percy's presence. This new entity could be extremely powerful and oppose Percy at every turn. It might be some sort of ancient, primordial being that has its own motives for not wanting Percy to succeed in whatever his mission in Tartarus is.
One possible plot development could be Percy facing off against new and terrifying monsters that are unique to Tartarus. Maybe he discovers some long - lost secrets about the place that could change the way the demigods view Tartarus in general.
One interesting plot could be May and Ash going on a special adventure together in a region they haven't explored before. They might encounter rare Pokémon and have to work as a team to overcome various challenges.
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.
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.