There could be new original antagonists created specifically for the fanfiction. For example, a secret organization that wants to use Naruto's lightning - controlling ability to power a dark and dangerous weapon they're building.
The Akatsuki members might also be antagonists. Since they're after the tailed beasts, Naruto with his additional power of lightning control would be a prime target. They might send their most powerful members like Pain or Itachi to deal with him.
It could be the Akatsuki members. Since they are often the main villains in the Naruto universe, they might also play that role in this fanfiction. They could be trying to stop Naruto from using his control over Sakura to achieve something that would be a threat to their plans.
It could be some of the traditional power - hungry ninja clans who oppose Naruto's new world order. They might see his attempts at global control as a threat to their own interests and try to undermine him.
It could be the Akatsuki. Since Naruto has this new power of controlling space, they might see him as an even greater threat and target him more aggressively. Their own powers and goals would likely clash with Naruto's new - found abilities.
Another way could be as a result of a special encounter. Maybe he discovers a hidden scroll or a mysterious artifact that imparts the power of lightning control to him. This discovery could be accidental, like when he's exploring an ancient ruin or during a battle when he stumbles upon something unexpected.
Well, the main antagonists could be a group of rogue ninja who are hired by a powerful nation to take down Naruto. Since he is unkillable, they would have to use more underhanded tactics like trying to isolate him from his friends and allies. Or perhaps there is a dark force that is awakened by Naruto's unkillable state, thinking that he is a threat to the balance of the ninja world. This dark force could then become the main antagonist, trying to destroy Naruto at all costs.
It could be an ancient evil that has been lurking in the shadows for ages. Maybe a dark spirit that was sealed away long ago and is now breaking free, and Naruto has to control the darkness within himself to fight it.
It could be Orochimaru. He's always been interested in new and powerful jutsus. He might see Naruto's shadow - controlling ability as something he wants to steal or use for his own evil purposes.
A powerful dark spirit might be the main antagonist. This spirit could sense Naruto's new ability to control darkness and see him as a threat. So it starts causing chaos in the ninja world, forcing Naruto to use his dark - controlling powers to stop it. The spirit could have the ability to possess other ninjas, making them do its bidding.
Maybe there are powerful dark sorcerers. These sorcerers could be trying to capture Naruto for their own evil purposes, like using his power to open a portal to a dark dimension or something, and Kurama has to protect him from them.
Another type of antagonist could be a strict professor who doesn't like Naruto's carefree attitude and Hinata's association with him. The professor might try to give them a hard time, like assigning them extra difficult projects or being overly critical in class, which becomes an obstacle in their college life and relationship.