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.
It could be Draco Malfoy. Their relationship would be full of tension as they've been rivals for a long time. But in the fanfiction, beneath the surface, there could be a hidden attraction that develops into love. And they keep it a secret because of their houses' enmity and their families' expectations.
One possible plot could be that Harry has to keep his relationship a secret because of the wizarding world's conservative views. He and his boyfriend might meet in secret places like the Forbidden Forest. They would have to be careful not to be discovered by their friends and classmates who might not understand their relationship at first.
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 could change the dynamics between the characters. For example, if Naruto's secret boyfriend was Sasuke, it would add a whole new layer to their already complex relationship. The reactions of other characters like Sakura would be very different. She might feel betrayed or confused.
Another character who might be her boyfriend in fanfiction is Viktor Krum. Given their connection from the Triwizard Tournament, it's not a stretch for fanfiction authors to develop a deeper romantic relationship between them. They could explore how Hermione's intelligence and Viktor's athleticism complement each other.
It could be Misty. They have a long - standing relationship in the Pokemon series. They've traveled together and shared many adventures, so in a fanfiction, it's not a stretch to imagine her as a secret girlfriend.
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.
A mysterious and brooding character would be great. For instance, a wizard who has been living in seclusion but is drawn to Sakura's positive energy. His magic and knowledge could add an exciting element to their relationship. He could teach Sakura about different spells and the magical world, and they could go on adventures together.
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.
In many fanfictions, Sakura could be a potential mother. Since she has a close relationship with Naruto in the original series, some fans might imagine a scenario where they end up together and have a son. Another possibility could be Hinata. Given her deep love for Naruto, it's not hard to fathom a fanfiction where they have a secret son.