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.
She could return with a new plan to redeem herself. Maybe she realizes the error of her ways after being away and tries to make amends with the ninjas. She might bring some new knowledge or power that could help in a new threat that's emerging in Ninjago.
One great Ninjago Harumi fanfiction is 'The Redemption of Harumi'. It shows her journey towards making up for her past wrongs in a very engaging way. The character development is well - written and it gives a new perspective on her role in the Ninjago universe.
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.
There could be a group of anti - ninja revolutionaries as the main antagonists. They see Naruto's return with the swords as a threat to their plan of overthrowing the existing ninja order. They try to take the swords from Naruto to use them as symbols of their new regime, believing that the power of the swords will help them convince other ninjas to join their cause.
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.
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.
It could be an unknown ninja who has been researching dark and forbidden arts. This ninja accidentally unleashes the Black Death - like plague while trying to gain ultimate power.
It could be a remnant of the Ice King's past enemies. Maybe one of the magical creatures that the Ice King used to annoy or fight against now sees Simon as a threat. Since Simon has the knowledge and power of the Ice King, this creature wants to stop him before he becomes too powerful again.
Perhaps an old enemy that Naruto thought was defeated has returned, stronger than ever. This enemy could hold a grudge against Naruto and the elemental nations for past defeats and wants revenge.
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.