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.
The main antagonist could be a powerful and corrupted spirit of the underworld. It has grown stronger with the chaos from the open Doors of Death and sees Percy as a threat to its newfound power. It uses all kinds of dark magic and minions to try and stop Percy from reaching the doors and closing them.
Maybe it's a new and vengeful demigod who was wronged by the gods. They believe that by keeping the Doors of Death open, they can force the gods to face the consequences of their actions. So, they do everything they can to stop Percy from closing the doors.
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.
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.
It could be a new, powerful and malevolent god. This god might be jealous of Percy's status as a hero and the favor he has from some of the Olympian gods. So, the new god devises a plan that makes Percy's life so miserable that he asks for death.
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.
It could be a power - hungry Atlantean noble who wants to rule not just Atlantis but also the surface world. He sees Percy as a threat to his plans and tries to stop him at every turn.
A new and powerful demon that has emerged from the darkest corners of the underworld. This demon is bent on using Percy's connection to the overworld to break through the barriers between the two worlds and unleash chaos. It has powers that are different from what Percy has faced before, like the ability to manipulate the very essence of the underworld's environment.
A powerful Amazonian sorceress who has turned against her own kind and is now trying to use dark magic to take over the world. She could see Percy as an obstacle in her path and constantly try to defeat him.
Maybe it could be a demigod who has become obsessed with power through gaming. This demigod, let's say a child of Hades, discovers a way to use dark magic and combine it with gaming technology to try and rule over all the other demigods. He sees Percy as a major threat to his plans because Percy is so popular and has strong powers, so he constantly tries to undermine Percy at every turn.
The goddess Hera might also be a great antagonist. Since she has a history of interfering in the lives of demigods, she could be jealous or threatened by Percy's rebirth. She might try to stop him from reaching his full potential in his new life or cause trouble between him and his new allies.
A powerful demigod who has been corrupted by the idea of primordial power could be the main antagonist. They might believe that by allying with a primordial or obtaining their power, they can become greater than the gods. This demigod would then oppose Percy, who stands for the protection of the current order.