It could be a long - lost relative of one of the old villains like Orochimaru. Maybe this person has been in hiding, training and growing in power, and now wants to take revenge on Konoha for what happened to their family member.
One possible plot could be that an ancient evil is awakened and sets its sights on destroying Konoha. Naruto, as the Hokage or a respected ninja, rallies all his friends and allies at first. However, as the battle progresses, he realizes that the only way to truly stop this evil is by giving his own life. So he makes the ultimate sacrifice, using his chakra to seal the evil away forever, thus protecting Konoha at the cost of his life.
It could be a corrupt elder in Konoha. They might be power - hungry and see Naruto as a threat to their control. So they manipulate the situation to make Naruto the enemy.
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.
It could be Orochimaru. Since Naruto was a potential threat to his plans in the original story, if Naruto died as a baby, Orochimaru might be able to carry out his experiments and schemes more freely without having to worry about Naruto interfering in the future.
It could be an unknown shinobi from a far - off land. A shinobi with powers that Naruto has never encountered before. This new antagonist might have some dark and powerful jutsu that Naruto simply isn't prepared for, which ultimately leads to his downfall on the bridge.
Maybe the main antagonist is a powerful figure within Konoha who doesn't want Naruto to leave and become an independent force. He secretly sends assassins after Naruto. When Temari gets involved, this figure also targets her. His actions are driven by his thirst for power and fear of Naruto's potential outside of Konoha's control.
Perhaps it's an old enemy who has somehow managed to come back stronger. Maybe someone from Orochimaru's old experiments that went wrong, but instead of dying, they gained some dark and powerful abilities. They see Naruto's return as a threat to their plans of world domination, so they target Konoha first. They might be able to control dark chakra in a way that no one has seen before, and use it to create chaos and destruction. They could also have some minions who are equally as powerful and loyal to them, making them a formidable foe for Naruto and Konoha.
The elders of Konoha could be the main antagonists. They might be the ones who have been neglecting Naruto all along, and when he leaves, they see him as a threat to Konoha's stability.