Some fanfictions also create new and original opponents for an abnormally strong Naruto. These could be powerful demons or entities from a different realm. They might be attracted to Naruto's newfound power and see him as a threat to their plans of taking over the Naruto universe. So they would try to eliminate him before he becomes too powerful to stop.
Another could be Kaguya Otsutsuki. However, in the fanfiction, she may return in a more powerful form. Maybe she has found a way to break free from the seals completely and has amassed even more power from other dimensions. Naruto, being abnormally strong, would still have a tough time fighting her as she is one of the most powerful characters in the Naruto lore.
One feature could be the creation of unique and overpowered jutsu for Naruto. For example, a fanfiction might give him the ability to control time on a large scale, which is far beyond his normal capabilities in the original series. This allows for really exciting and unpredictable plot twists.
Often, it's powerful villains like Orochimaru who make a comeback in fanfiction. Naruto has to use his growth and new abilities to defeat him once and for all.
Often, it's other powerful ninjas from rival villages. For example, shinobi from the Hidden Mist Village might be opponents. They could have unique abilities that Naruto has to overcome to beat them up.
This is a Xianxia fantasy immortal cultivation novel. The protagonist, Yang Jie, transmigrated to the Xianxia plane. In order to return to his original plane, he began an extraordinary path of demonic cultivation and tomb raiding. It was very interesting and suitable for readers who liked the main characters of demonic cultivation. I hope you like my recommendation. Muah ~
Sasuke would still be a likely opponent. Even though Naruto has enhanced speed, Sasuke has his own unique abilities like the Sharingan which can predict movements to some extent. Their battles would be even more intense, with Naruto's speed against Sasuke's strategic use of his Sharingan and ninjutsu.
Often, he overcomes powerful villains like Madara Uchiha. In some fanfictions, Madara might be even more powerful than in the original, but Naruto's newfound abilities allow him to defeat him. Another could be Orochimaru, who might have some new and dangerous experiments, but Naruto is still able to stop him.
Kakashi could also be an opponent. He would be a very challenging one, using his vast knowledge and experience. He might teach Naruto some chess strategies during the game, just like he teaches him ninja techniques.
Superman can also be an opponent in some 'death battle batman fanfiction'. It's an interesting match - up as it pits Batman's brains and preparation against Superman's superhuman strength. Writers often explore how Batman could potentially defeat someone so much more powerful.
One common theme is power - up. Usually, Naruto gets some sort of new power or ability that makes him much stronger. Another theme is rivalry. There are often new rivals for Naruto who are also very strong, which drives him to keep getting stronger. And there's also the theme of self - discovery, as Naruto figures out how to use his newfound strength.
Voldemort is a common one. In many fanfictions, their duels are epic battles of good vs evil, with Harry using his duelist skills to outwit Voldemort.