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.
A possible plot twist could be that his secret fiance is actually an enemy in disguise. She got close to Naruto under false pretenses, and as the story unfolds, Naruto has to deal with the betrayal and the fact that his feelings were used against him.
Naruto and Rias are the central characters. Naruto is known for his never - give - up attitude and his array of ninja techniques. Rias is a complex character with her own political and social standing in the devil world. However, in some fanfictions, other characters from both the Naruto and High School DxD universes can become important. Kiba from Naruto might befriend Naruto in the new world and help him understand the devil society better. And from the High School DxD side, Akeno, Rias' friend, could play a role in either supporting or testing Naruto's relationship with Rias.
At first, they might be a bit cautious around each other. But as they face challenges together, such as dealing with rogue shinobi who don't want peace, their relationship deepens. Naruto's positive attitude rubs off on Mei, and Mei's practical thinking helps Naruto. They could end up being very close allies, with a bond that is based on trust and shared goals of maintaining peace in the shinobi world.
Well, in the beginning, their relationship would be more like a business deal for the sake of their villages. But Naruto, being Naruto, would try to make the best of it. He would start conversations with Mei about simple things like food or the weather in their respective villages. Mei, at first, might be a bit dismissive but gradually she would be intrigued by Naruto's positive energy. As time passes, they would be involved in more serious matters together, like dealing with rogue ninjas or negotiating treaties. Through these shared experiences, their relationship would evolve from a forced political union to a deep emotional connection where they truly care for each other.
Enemies from Naruto's past or Sona's enemies could target them as a couple. Since Naruto has made many foes during his adventures, and Sona may also have her own enemies, their engagement could put a target on their backs.
Perhaps in this fanfiction world, political alliances play a role. Naruto, being a powerful and respected figure, and Sona, having her own influence, their marriage is arranged to bring peace between two regions or groups. It could also be that some sort of magical or supernatural force has brought them together and fated them to be engaged.
A common plot in Naruto Rias Fiance fanfiction is the power - up journey of Naruto. As he is now in a new environment with different power systems, he has to level up. He could gain new devil - related powers through his connection with Rias. Also, there could be a plot about their relationship development. How they overcome misunderstandings and cultural differences. For instance, Naruto's straightforward ninja way might clash with the more complex and often secretive devil society, and how they resolve these issues can be an exciting part of the story.
Well, it's highly likely. Fanfiction is a place where people can play with established canon and create their own versions. Given Mei Terumi's character from the Naruto universe, someone could have written a fanfic where she is Naruto's mother. It could lead to some really interesting changes in Naruto's story. Maybe her different upbringing and values as a Kage from a different village would influence Naruto in different ways. And it would also change the interactions Naruto has with other characters, like how he relates to the Hidden Mist Village if his mother is from there.
Mei might start as a relatively minor character in the fanfiction. But as she gets involved in Naruto's mission to seal the Juubi, her character could grow. She could become more confident in her abilities and start to take on more leadership roles. For example, she might lead a group of ninjas to protect Naruto while he is performing the sealing. This experience could make her more respected in her own village and also in the wider ninja world.