Another plot could be that something from Tartarus has followed Percy out. A small, malevolent creature that starts causing trouble in Camp Half - Blood. Percy feels responsible for it and has to find a way to send it back to Tartarus without putting everyone in danger.
A great plot could involve Percy finding a way to use the unique environment of Tartarus to his advantage. For example, there could be some sort of energy in Tartarus that he discovers he can harness to become stronger, but it has a dangerous side - like it might slowly corrupt him. He then has to struggle with the decision of whether to keep using it to get out or resist it.
In a Percy Jackson Tartarus fanfic, a great plot could be centered around Tartarus corrupting the very essence of Percy's powers. He starts to lose control of his water - based abilities and instead they become tainted with the darkness of Tartarus. Percy then has to go on a journey within Tartarus to find a way to cleanse his powers, all while being hunted by the most dangerous creatures that lurk in the abyss.
A really cool plot could be that Percy is alone in Tartarus and he starts to lose his memory due to the dark magic there. As he forgets who he is, he still has his instincts. He begins to form an unlikely alliance with some of the less evil creatures in Tartarus in order to survive. He has to then rediscover his identity while trying to find a way out of this hellish place.
Well, a possible plot is that Percy has a flashback to a time in Tartarus when he witnessed a prophecy being fulfilled in a way he didn't expect. This could lead him to re - evaluate his understanding of prophecies in the present. Another idea is that his flashback reveals a secret passage or power source in Tartarus that he believes might exist in the mortal world too, and he sets out to find it. Also, the flashback could be about a time when Percy was separated from Annabeth in Tartarus for a brief moment, and that fear of losing her again triggers his current actions to protect her more fiercely.
They could discover a lost civilization in Tartarus. This civilization might have a way to manipulate the very fabric of Tartarus itself. Percy and Annabeth would then have to learn the ways of this civilization in order to use it to their advantage. Maybe they have to earn the trust of the civilization's leaders by helping them with their own internal problems first.
One idea could be Percy having to prove himself in the Underworld. Since he's the son of Tartarus, he might face unique challenges from the other denizens who fear or distrust him. Maybe he has to compete in some sort of underworldly tournament to gain respect.
One plot idea could be Percy struggling with his new identity. Since Tartarus and Nyx are such powerful and dark entities, Percy might have to battle against the dark urges within him. He could be shunned by his friends at first, but then prove that he's not defined by his parentage. For example, he might save Camp Half - Blood from a new, powerful threat using his unique powers that come from his heritage.
One possible plot twist could be that Percy discovers a hidden power within Tartarus that he can use to his advantage. Maybe it's a long - lost magic that only the darkest part of the Underworld can unlock.
Another plot twist might be that Tartarus itself has a weakness that Percy discovers by accident. For example, he might notice that certain sounds or vibrations can disrupt the structure of Tartarus. This would completely change the game as he could then use this knowledge to his advantage, perhaps causing mini - earthquakes to distract or even harm the monsters that are after him.
Well, perhaps a plot twist is that Percy doesn't escape in the traditional sense. Instead, he finds a way to manipulate the rules of Tartarus so that he can exist in both Tartarus and the mortal world simultaneously. This could lead to some really interesting story developments as he tries to keep this a secret from his friends while also using his new - found ability to fight against the forces of evil. Also, it could be that the escape route he finds is actually a trap set by an enemy, but he manages to turn it around and use it to his advantage in a really unexpected way.