It could be Orochimaru. He's always been a tricky and power - hungry character. Maybe while Naruto was in stasis, Orochimaru found a way to gain even more power and now he's a greater threat than ever. He might be after Naruto's new - found power that he got during his stasis or some secret knowledge that Naruto has.
In the 'naruto awakens from statis fanfiction', it could be that Naruto wakes up with amnesia. He doesn't remember who he is or what his powers are. As he tries to figure things out, he meets new people who either want to help him or take advantage of his situation. He slowly starts to regain his memories through various encounters and battles. This would be an interesting twist as we get to see Naruto rediscover himself and his place in the ninja world.
It could be a new character who is jealous of Naruto's newfound power of the Rinnegan. Maybe someone who has been in the shadows all along and now sees Naruto as a threat to their plans for world domination.
It could be a new character, a rogue ninja from a long - lost clan. This antagonist might have a vendetta against Naruto because they see his potential to awaken the Rinnegan as a threat to their own plans for power. They could be trying to stop Naruto from achieving this new level of power so that they can carry out their evil deeds unopposed.
It could be a rogue ninja from a forgotten clan who has a vendetta against the Hidden Leaf. This ninja might be seeking to obtain the power of the Mangekyou Sharingan for himself, and sees Naruto as a threat. So, he tries to stop Naruto from fully awakening it and uses all kinds of dark jutsu to attack Naruto and his friends.
There's also the possibility that the main antagonist is a group of ninja from a rival village. They fear Naruto's new power and form an alliance to take him down. They might have spies within Naruto's village trying to gather information about his kekkei genkai so they can develop counter - measures and ultimately defeat him.
A new character from the Kingdom Hearts universe could be the main antagonist. Maybe a powerful Heartless or a dark Keyblade wielder who sees Harry as a threat since he has awakened the keyblade. This antagonist could be after Harry to take the keyblade for their own evil plans.
It could be a powerful Quincy who sees Ichigo's newly awakened powers as a threat. This Quincy might want to eliminate Ichigo before he can fully master his powers and use them against the Quincy community.
It could be Voldemort still. Since Harry has awakened the devil gene, Voldemort might see this as an opportunity to either corrupt Harry further or use his new power for his own dark purposes. So he would be the main force against Harry.
It could be a rogue ninja from within Konoha who has embraced the 'darkness' fully and wants to reshape the village according to his own twisted ideals.
It could be a rogue ninja from his own village who wants to use Naruto's unique position (being from a different village but having ties to the Leaf through his adventures) to start chaos. This ninja might be jealous of Naruto's popularity and abilities, so he tries to frame Naruto for various crimes to turn the different villages against him.