It could be Hinata. In the original series, Hinata has a deep affection for Naruto. In a fanfiction, it would be a natural progression for her to be his secret fiance. Maybe they got engaged secretly to avoid some political unrest in the village.
It might also be Gaara. Their initial encounters were filled with tension, but as the story could be twisted in fanfiction, that could develop into a different kind of relationship. Gaara's own journey of self - discovery and Naruto's influence on him could be a basis for a romantic relationship in fanfic.
Well, it could be a fanfiction where Naruto discovers he has the Mokuton ability in a very unexpected way. Maybe he gets it through some ancient power transfer or a connection to a long - lost clan. And this new power of Mokuton could lead him to face new enemies or form new alliances. It might also explore how his friends react to this new side of him.
It could be that Naruto is secretly training a new and powerful jutsu that he doesn't want anyone to know about yet. Maybe he's afraid that if his enemies find out, they'll try to stop him before he perfects it.
Naruto might team up with Gaara. Gaara has his own unique abilities with sand control. Together, they could form a strong duo for a secret mission, especially if it involves something related to the balance of power between their respective villages.
Well, it could make Naruto extremely powerful right from the start. With the Rinnegan, he might be able to master all types of jutsu more quickly. He could also have an edge in battles against powerful foes like Madara or Kaguya. Maybe he'd be able to use the six paths techniques with more ease and less training.
It could change the relationships Naruto has with other characters. For example, his friends might be shocked at first but then become protective of his daughter. His enemies could see her as a new target or a way to get to Naruto.
It could be that a new character, someone from a far - off land that Naruto met during one of his travels, is involved in his secret life. This character could bring with them a different culture and set of skills that Naruto is learning from in secret, perhaps something that could revolutionize the way ninjas in the Hidden Leaf operate.
It could be a random villager that Naruto met during one of his travels. Maybe she was from a small village that Naruto helped protect, and they had a brief encounter that led to the child.
One possible plot could be that Naruto discovers he has a child during a mission. Maybe he saved a village years ago and unknowingly had a relationship with a local woman. The child could be in danger from some villains who want to use the child's connection to Naruto for their own evil plans.