Who could be the main antagonist in a 'Pertemis Percy Dies' fanfiction?

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.

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.

Hawkmoth could be the main antagonist. In the original story, he's always causing trouble. In a 'marinette dies' fanfic, he could finally succeed in his evil plan, maybe by getting his hands on the Ladybug and Cat Miraculouses and using their combined power against Marinette.

It could be an ancient and powerful god or goddess who has a grudge against Percy. Maybe they see Percy as a threat to their power or plans, so they plot his death. For example, a forgotten deity who wants to regain their former glory by getting rid of Percy, who is often seen as a hero in the Percy Jackson universe.

It could be a local Siberian warlord who sees Tony as a threat to his power. Maybe Tony's presence in Siberia disrupts the warlord's plans for the region, so he decides to eliminate him.

A new and powerful ghost that Danny has never faced before could be the main antagonist. This ghost could have unique powers that Danny isn't prepared for, and in their first encounter, it manages to defeat and kill Danny. This would set the stage for a story of revenge or trying to bring Danny back.

It could be a new character created just for the fanfiction. Maybe a rogue ninja who has mastered a dark and powerful jutsu that Naruto has no defense against. This antagonist could be seeking revenge on Konoha for some past wrong and sees Naruto as the biggest threat to his plans.

It could be someone who was secretly behind Naruto's death in the first place. Maybe a rogue ninja who has a vendetta against the Hidden Leaf and saw Naruto as a major obstacle.

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.