Maybe it's a powerful and corrupted demigod. This demigod could be jealous of Percy's fame and power. He devises a scheme to kill Percy and take his place. However, Percy's unexpected rebirth throws a wrench in his plans. Now, the resurrected Percy has to face this traitorous demigod and put an end to his evil ways.
It could be a new Titan that has been awakened from an ancient slumber. This Titan might be angry at the gods for imprisoning them long ago and wants revenge. Percy's death could be part of the Titan's initial plan to strike fear into the hearts of the gods and demigods, but Percy's rebirth becomes a major obstacle for the Titan.
It could be a new, powerful Titan that was previously unknown. This Titan might have a vendetta against Percy specifically, seeing him as a major threat to their plans. Maybe they manage to kill Percy, but his rebirth sets in motion a series of events that will lead to their ultimate downfall.
It could be a powerful Titan who has escaped from Tartarus and blames Artemis for something in the past. So he's out to get her, and Percy has to stop him at all costs, even if it means his death.
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.
Well, perhaps he could die sacrificing himself to save his friends from some ultimate evil. Then, his rebirth could be linked to a powerful ancient magic. Maybe a forgotten god decides to bring him back for some grand purpose, like to stop a new threat emerging in the world of the gods.
It could be a jealous demigod. Maybe someone who has always been envious of Percy's popularity and power. This demigod could spread lies about Percy that lead to his disownment, making them the main antagonist.
It could be an old enemy from Percy's past life who has also found a way to be reincarnated. This enemy might hold a grudge against Percy for something that happened in their previous encounter and is now seeking revenge in this new life.
Perhaps a powerful and corrupted demigod could be the main antagonist. This demigod might be jealous of Artemis' power and popularity among the other demigods. He devises a plan to harm her, and Percy, being loyal to Artemis, has to give his life to save her from this rogue demigod.
A powerful Titan who has escaped from Tartarus. This Titan blames Percy for his previous imprisonment and wants to make Artemis suffer as well for her role in keeping the Titans locked away. He uses Percy's death as a way to manipulate the situation and turn the demigods against Artemis, or he might try to steal Percy's powers that were left behind after his death for his own evil purposes.
It could be an ancient evil that was sealed away long ago but has been released due to Naruto's death. Maybe Naruto's sacrifice in his previous life weakened the seal, and now this evil has emerged. It could be a powerful demon or a dark ninja with extremely strong and malevolent jutsu.
Another option is a group of anti - magic fanatics. They view Percy's wings as an abomination because they are a result of magic. They hunt Percy down, believing that by getting rid of him and his wings, they are protecting the world from the 'evils' of magic.