One idea could be Percy having to face his inner demons while closing the Doors of Death. Maybe past failures or losses resurface, adding an emotional depth to the task.
A possible plot could be that the Doors of Death are guarded by a powerful entity that is corrupted by the power it wields. Percy has to not only defeat this entity but also find a way to purify the area around the Doors of Death so that they can be closed for good. He might have to use his wits and his powers in ways he hasn't before.
Well, it could involve Percy facing numerous challenges in the underworld. Maybe he has to battle new and powerful monsters that guard the Doors of Death. He might also get some unexpected help from his friends, like Annabeth using her wisdom to figure out the secrets of closing the doors. And there could be a final showdown with some major villain who doesn't want the doors to be closed.
Maybe a group of rogue demigods who have been corrupted by dark magic. They believe that by keeping the Doors of Death open, they can gain immortality or unlimited power. Percy would then have to face his own kind in order to close the Doors.
It could be an ancient and powerful Titan who escaped from Tartarus when the Doors of Death started to malfunction. This Titan wants to keep the doors open to bring back all the old enemies of the gods and take over the world again.
One idea could be Percy sacrificing himself to save his friends and Camp Half - Blood from a powerful new enemy. Another could be a wrong - turn in a quest that leads to his untimely death and how his friends cope with the loss. Maybe in some fanfictions, Percy's death is part of a prophecy that no one could avoid.
A cool plot could be that the God of Death has gone rogue and is starting to take souls before their time. Percy, with his friends, has to find a way to stop the god. They would have to search for ancient artifacts that can control or at least influence the God of Death's actions. They'd face many challenges, like traps in the Underworld and battles with deathly minions.
One plot idea could be Percy having to team up with Hera to rescue a group of demigods who have been captured by an ancient monster. Hera, despite her usual aloofness, realizes she needs Percy's unique skills.
One idea could be that Percy is framed for stealing a powerful magical artifact. His friends initially doubt him, but as he tries to prove his innocence, he uncovers a conspiracy within the magical community. For example, a secret group wants to use the situation to gain more power and is setting him up as a scapegoat.
One idea could be that Percy has to seek Athena's wisdom to solve a complex mystery in the mortal world that is somehow related to the gods' ancient secrets. Maybe it involves a powerful artifact that was lost long ago and only Athena's knowledge can lead Percy to it.
One idea could be Percy joining the hunt temporarily. Maybe there's a prophecy that requires his unique skills and the hunt needs his help to complete a mission. For example, a powerful monster is on the loose that only Percy can track down because of his connection to the sea. Another could be a love triangle between Percy, a huntress, and someone from Camp Half - Blood.
One plot idea could be Percy discovering he has the ability to bloodbend and struggling to control it. He might be afraid of the power and how it goes against his usual water - based abilities. Another could be Percy having to face an enemy who uses bloodbending in a malicious way, and he has to learn it quickly to defend himself and his friends.