Maybe Sakura. Although she has feelings for Sasuke in the original, in fanfiction, things can be different. Naruto might secretly marry Sakura because they've been through a lot together and developed a deep bond that they keep hidden from others for some personal or external reasons.
There's also a possibility he could marry a new character created by the fanfiction writer. This new character could be from a far - off land and they fall in love in secret. Naruto would then marry her in a private ceremony to keep their relationship away from the chaos and politics of the ninja world.
He could secretly marry Hinata. In the original series, they have a connection, so in fanfiction, it could be a scenario where they elope or have a secret ceremony for some reason, like to avoid political interference in their relationship.
It could be a rogue shinobi from Naruto's past who has been biding his time. This shinobi might have developed a new jutsu specifically designed to target Naruto and make him die slowly. He's doing it secretly to cause chaos in the Hidden Leaf when their hero suddenly passes away.
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.
Perhaps an old enemy of the Sannin resurfaces. This antagonist could see Naruto's attempts to follow in the Sannin's footsteps as a threat. They might target Naruto to get revenge on the Sannin or to prevent a new powerful force from emerging in the ninja world. For example, if there were a group that was once defeated by the combined efforts of the Sannin, they could try to stop Naruto before he becomes as strong as the Sannin were together.
It could be someone who wants to expose Naruto's secret. Maybe a rogue ninja who wants to use this information to create chaos in the village by making the villagers doubt Naruto's intentions.
One reason could be to protect the person he marries. If she's a commoner, for example, the noble society in Camelot might not accept their relationship. So, they keep it a secret. Another reason could be that the person he marries has some sort of power or identity that could put them in danger if it was known. For instance, if she was a sorceress, in Camelot where magic is feared, they'd have to marry secretly.
Sakura could be a potential partner. Since they are friends from the original series, a forced marriage situation between them would bring a whole new dynamic. There would be a lot of room for exploring their relationship beyond just friendship.
He could marry Mikasa from Attack on Titan. Just think about it. Naruto's positive and determined personality would contrast with Mikasa's more stoic and protective nature. They could meet through some sort of interdimensional accident. Naruto would be fascinated by the Titans and the world outside his own, and Mikasa might be intrigued by Naruto's ninja abilities.
He could marry Hinata. Even though he's banished, Hinata might follow him because of her deep love for him. Their relationship could develop further outside the village, and they could get married and start a family.
In some fanfictions, it could be due to a prophecy or a magical bond that they discover. They might feel that the only way to fulfill the prophecy or strengthen the bond is by getting married secretly. Another aspect could be the political situation in the wizarding world. There could be political factions that would oppose their union for various reasons, such as Harry being seen as a symbol of a certain side and Fleur being from a different background. So, to avoid political turmoil or to keep their relationship safe from political manipulation, they choose to marry secretly.
His friends would also be affected. They might be worried about Naruto's well - being. They know him well and might be concerned that his refusal to marry is a sign of some deeper emotional issue. They would want to support him but also might try to persuade him to consider marriage in the future.