It could be a jealous demigod who thought that with Percy dead, they would be the most powerful or important one. Now that Percy is back, they see him as a threat to their status and plan to get rid of him again.
Maybe it's a new character who is jealous of Percy's status among the demigods. This new antagonist could be a powerful demigod from a different camp who has heard of Percy's feats and doesn't want him to steal the spotlight again. So, they try to use Percy's return from the dead as a way to turn the other demigods against him.
A new dark entity could be the main antagonist. This entity might have been awakened by Naruto's return from the dead. It could be something that was sealed away long ago and Naruto's return breaks the seal. The entity would then target Naruto and his loved ones to prevent him from getting in its way of ruling the ninja world.
A group of rogue demigods could be the main antagonists. They believe that Percy is too powerful and his presence is causing more problems than solutions in the demigod world. So they make him disappear. But when Percy comes back, he shows them that they were wrong and that they have been mislead by false beliefs.
One possible plot could be that Percy returns with new powers. Maybe he was resurrected by some ancient magic or a powerful deity who has a plan for him. He then has to deal with the shock and awe of his friends and also face new enemies who are either threatened by his return or want to use his newfound powers for their own gain.
Another option is that there is a group in his own world that doesn't like the idea of someone coming back from another world. They might see Izuku as an anomaly and try to capture or study him. This group could be a secret organization that has its own motives, and in their attempts to deal with Izuku, they end up being the main antagonists.
One possibility could be that Percy returns with new powers. Maybe he has some connection to the Underworld now that he's come back from the dead. He might be able to control the dead or has some knowledge about the afterlife that he uses to help his friends in battles against new and more powerful enemies.
It could be a new villain who has risen to power during Naruto's absence. This villain might have a vendetta against Konoha or the ninja world in general because of something that happened while Naruto was away.
The main antagonist could be an ancient evil that has been awakened. Harumi returns to find Ninjago in chaos because of this new threat. She may initially be blamed for the chaos, but as the story unfolds, it becomes clear that the real villain is this ancient evil which the ninjas and Harumi must then team up to defeat.
It could be the warden of the prison. The warden might be a cruel and power - hungry person who wants to keep Percy imprisoned for his own reasons, like getting some sort of reward or power from those who put Percy there.
A powerful dark sorcerer could be the main antagonist. He might be seeking to use Percy's skills for his own evil purposes, and when Percy refuses, he becomes the target. The sorcerer would have a vast array of dark magic at his disposal, making him a formidable foe.