Well, it could be Hinata. In the original series, they have a strong connection, so in a fanfiction, it's quite possible that Naruto would choose her to propose to.
Another possibility is that he asks someone completely unexpected, like Tenten. In fanfiction, authors often like to take creative and unexpected turns. Tenten might have had some adventures with Naruto that led him to see her in a new light and decide to ask her to marry him.
If he asks Hinata, she would probably be overjoyed. She has always been in love with Naruto, so her answer would most likely be an immediate 'yes' with tears of happiness in her eyes.
Well, it could be a fantasy fanfiction where the angel has powerful magic to shield Naruto from all kinds of threats.
Well, it really depends on the writer's preference in the fanfiction world. If the writer wants to focus on the initial crush Naruto had on Sakura, they might make Naruto and Sakura get together. However, if they are more into the idea of Naruto finally realizing Hinata's true feelings and reciprocating, then Naruto will be with Hinata.
Well, it could be due to some powerful magic or trickery by Rias. Maybe she has some ulterior motives and uses her special abilities to enslave Naruto. But this is just speculation as it depends on how the fanfiction writer has crafted the story.
One reason could be for power. Combining their strengths through marriage could make them an even more formidable force. Maybe Sasuke saw it as a strategic move to protect their village or to face some greater threat that they are not aware of yet.
It could be a character from another important ninja clan, like Hinata perhaps. Since Hinata has always had feelings for Naruto and her clan is also a significant part of the village, Minato might see it as a good match.
Another possibility is Sakura. Although she has feelings for Sasuke in the original, in this fanfiction, the story could be rewritten so that she realizes her true connection with Naruto and they end up getting married at 12. Their dynamic would be quite different from the original, with Naruto having to prove himself in a new way to Sakura.
Yes, Naruto does end up with Hinata in the manga. Their relationship develops over time and culminates in a happy ending together.
Sometimes, in fanfiction, Naruto stands up to Sakura when it comes to her attitude towards Sasuke. Sakura might be blindly following Sasuke's every move, and Naruto, having a more objective view of Sasuke, could confront Sakura. He would tell her that she should value herself more and not just revolve her life around Sasuke. Naruto has his own understanding of friendship and loyalty, and he would use that to make Sakura see things differently.
Well, one possible plot could be that after years of fighting side by side and having a deep understanding of each other, Sasuke finally realizes his true feelings for Naruto are more than just friendship. He might plan a very private and special moment to pop the question. Naruto, being the open - hearted person he is, would be initially shocked but then consider it seriously as he also values Sasuke deeply.